1)
What is the difference between a Mizbe'ach, a Matzeivah and an Asheirah?
1.
Rashi, Targum Onkelos and Targum Yonasan: A Mizbe'ach is an altar made of many stones and a Matzeivah, one made of one stone; whereas an Asheirah is a tree that is worshipped.
2)
Seeing as the Torah discusses the Mitzvah of burning Asheiros in Re'ei - Devarim, 12:3 - why does it mention cutting them down here?