Gemara

"Me'Asos Derachecha" -- your should not walk on Shabbos the same way you walk on a weekday;

"V'Daber Davar" - your speech on Shabbos should not be like your speech on a weekday;

"Davar" - speech is forbidden, but thoughts are permitted.

(Beraisa): Reuven may not tell Shimon 'come to me at night' [so I can hire you];

R. Yehoshua ben Korchah permits.

(Rabah bar bar Chanah): The Halachah follows R. Yehoshua.

(R. Yochanan): R. Yehoshua expounds "mi'Mtzo Cheftzecha v'Daber Davar" - speech [that explicitly discusses Melachah] is forbidden, but thoughts are permitted.

Question: Speech is not forbidden!

(Rav Chisda and Rav Hamnuna): We make calculations of a Mitzvah on Shabbos.

(R. Elazar): We may apportion Tzedakah to the poor on Shabbos.

(R. Yakov): On Shabbos we go to Batei Kenesiyos and Batei Medrashos to oversee communal concerns.

(R. Yakov bar Idi): We oversee mortal issues on Shabbos.

(R. Menasiya): On Shabbos one may arrange Kidushin for his daughter and teachers to teach Torah or a trade to his son.

Answer: "Mi'Mtzo Cheftzecha v'Daber Davar" - your pursuits are forbidden, but those of Shomayim (Mitzvos) are permitted.

(Rav Yehudah): On Shabbos one may make calculations that pertain to the king (not to the one doing them), or that have no importance for anyone.

Support (Beraisa #1): (On Shabbos) one may not make calculations that were once relevant or will be relevant to him;

(A commoner) may make calculations relevant to the king or that have no relevance to anyone.

It is permitted only if he finished paying the workers.

150b (Rav Yehudah): On Shabbos, one may say 'I will go to city Ploni' [even if it is outside the Techum], because if there were shacks [every 70 Amos along the way to extend the city], it would be within the Techum.

Rishonim

Rif and Rosh (64a and 23:4): One may say to a worker 'come to me at night.'

Ran (DH Gemara): Even though both understand that he wants to hire him, it is permitted because he did not explicitly say so. Speech is forbidden, but thoughts are permitted.

Rif and Rosh (ibid. and 23:5): Calculations that have no importance for anyone or for the one making them may be made on Shabbos. Calculations that were once relevant to him are permitted only if they are no longer relevant.

Rif and Rambam (ibid. and Hilchos Shabbos 24:3): One may say 'I will go to city Ploni', because if there were shacks, one could go there on Shabbos.

Rosh (ibid.): If something could be permitted on Shabbos, one may talk about doing it or ask his friend to do it.

Rambam (23:18): One may not make calculations that he needs, whether they passed or are still relevant. This is a decree lest he write. Therefore, calculations that have no importance are permitted, e.g. how much grain he grew last year or he spent on his son's wedding. This is idle talk. There is no difference between such calculations on Shabbos and during the week.

Magid Mishneh: Idle talk is not proper for one who fears sin. Too much idle talk on Shabbos is forbidden to everyone (the next Halachah).

Rambam (24:4): One may say to a worker 'come to me at night'. He may not say 'be ready for me at night', for that is pursuing his needs on Shabbos. It is forbidden to talk too much idle talk on Shabbos, for it says "v'Daber Davar" -- your speech on Shabbos should be unlike your weekday speech.

Rosh (15:2): V'Daber Davar" teaches that your speech on Shabbos should not be like your speech on a weekday. One may not say excessive idle words. Vayikra Rabah (34) says that when R. Shimon's mother would talk too much [on Shabbos], he would say 'it is Shabbos.' The Yerushalmi says that with difficulty, they permitted greeting people on Shabbos.

Terumas ha'Deshen (61): Tosfos and the Rosh forbid excessive idle talk on Shabbos, like the case of R. Shimon's mother. After he said 'it is Shabbos', she would be quiet. This connotes one should not talk on Shabbos as much as during the week. All the more so one may not talk more than during the week! However, if people enjoy stories of kings, wars, etc., surely it is permitted. This is like Semak, who permits youths who enjoy running and jumping, and to see something one enjoys seeing. Even though Chachamim expounded that your walking on Shabbos should be unlike your walking on a weekday, i.e. one may not run and jump, one may do so for enjoyment. The same applies to excessive talk, which we learn from the same verse. However, I saw that many people who gather to tell these stories do not enjoy them, just they tell them to please their friends gathered with them. There is concern for an Isur for those who do not enjoy them.

Sefer Chasidim (266): On Shabbos one may not say 'let us sleep, so we will be able to do our work on Motza'ei Shabbos.' This is even if he wants to write Divrei Torah, for he shows that his rest on Shabbos is for the sake of a weekday. Rather, he says 'let us rest, for today is Shabbos.'

Poskim

Shulchan Aruch (OC 307:1): "V'Daber Davar" teaches that your speech on Shabbos should not be like your speech on a weekday. Therefore, one may not say 'tomorrow I will do matter Ploni, or buy merchandise Ploni.'

Magen Avraham (1): One may not even talk about doing a Mitzvah tomorrow (Sefer Chasidim). The Agudah forbids detaining a Nochri (Tony) on Shabbos due to a debt that Tony owes him. However, during the week he may say to the [Nochri] judge 'why didn't you detain Tony on Shabbos?' One may not withhold Reuven's property in the hands of Shimon [to help Shimon collect from it].

Mishnah Berurah (1): This is only if the matter is forbidden, even if it is an Isur mid'Rabanan. Some forbid even for a Mitzvah like writing a Sefer Torah. However, Eliyahu Rabah, Ma'amar Mordechai and Birkei Yosef permit any Devar Mitzvah. In any case l'Chatchilah one should be stringent if there is no need to talk about it today. If he fears lest he slacken, all permit to make himself zealous. Saying to Hash-m [that he will do a Mitzvah] is like a vow. Chachamim did not decree if it will cause a loss. If a Nochri (Tony) who owes him money will leave permanently on Shabbos, he may tell an officer or judge to make Tony pay another Nochri or get a guarantor before he leaves.

Eshel Avraham (1): Sleeping on Shabbos in order to write a Sefer Torah at night is not a need of Shabbos. One may say 'I will write a Sefer Torah on a Yom Chol.' This requires investigation.

Mishnah Berurah (6): People of great deeds are careful to speak even necessary matters only in Lashon ha'Kodesh on Shabbos. This is a fence, lest they come to idle talk. The Shlah says that one who greets another on Shabbos morning should not greet like he does during the week 'good morning', rather, he should say 'good Shabbos' [in Leshon ha'Kodesh or Leshon Chol, like the custom nowadays], in order to fulfill "remember the Shabbos day."

Kaf ha'Chayim (1): The Ari Zal would speak another language on Shabbos or Yom Tov only if necessary, to explain something to Talmidim.

Kaf ha'Chayim (3): One may discuss a Devar Mitzvah.

Shulchan Aruch (ibid.): One may not even say too many idle words.

Gra (DH Leharbos): It would be permitted if not for this [excess. I.e. we permit a small amount of idle talk.] The Yerushalmi and Rambam connote like this. Also Sa'if 6 connotes like this. [It permits calculations] if they were already paid. The Rambam calls this idle talk.

Mishnah Berurah (2): This refers to words not connected to Melachah or business, and there is no detriment [to a Yisrael] or frivolity. If not, even a small amount is forbidden!

Kaf ha'Chayim (11): The text of the Shulchan Aruch should say 'he should not say too many Chulin words.

Kaf ha'Chayim (4): One may not proofread in order to fix mistakes tomorrow. One may read, and if he finds a mistake, he will fix it tomorrow.

Rema: People who enjoy talking about things they heard and news, they may say them on Shabbos like during the week, but one who does not enjoy them may not say them in order that his friend will enjoy them.

Taz (1): The Rema should have written 'even one who does not enjoy...' (Mishbetzos Zahav - the Taz' text of the Rema said '... but one who does not enjoy them may say them.) The Levush says '... but one who does not enjoy them may not say them', and so it says in Terumas ha'Deshen. I say that if they intend to enjoy telling the stories, there is no concern. He enjoys that his friend enjoys it.

Mishnah Berurah (6): One should not transgress v'Daber Davar for the sake of his friend. Perhaps one may be lenient if he gets Oneg from their Oneg.

Mishnah Berurah (3): On Shabbos one may not tell a matter that pains him.

Kaf ha'Chayim (7): One may not talk about his beloved who died or is in pain.

Mishnah Berurah (4): Even in a matter that is absolute Oneg, e.g. eating and drinking or sleeping, one should not spend too much time. For people who do not learn Torah during the week, Shabbos was given primarily for Divrei Torah. Even for Chachamim who toil in Torah the entire week, they should not spend too much time [on Oneg] due to Bitul Torah.

Mishnah Berurah (5): This is not for Chasidim to do. One who refrains from this is called Kadosh.