The Scapegoat

The he goat shall thus carry upon itself all their sins to a precipitous land, and he shall send off the he goat into the desert. [Acharei 16:22]

For what sins does the scapegoat atone?

Rambam: The scapegoat atones for the entire Jewish people...for all transgressions of the Torah, both severe and less severe sins; those violated intentionally and those violated unintentionally, whether the person was aware of his sin or not - all are atoned for by the scapegoat.  But this is provided that one does teshuvah.  If one does not do teshuvah, the goat atones only for less severe sins.

Which sins are considered "severe" and which are considered "less severe"?

The "severe" sins are those for which a person is liable either for execution by a court or soul excision (kares)... Other prohibitions and all positive commands that are not punishable by soul excision are "less severe sins".

Now that the Temple no longer exists and there is no Altar to atone, there is only teshuvah, and teshuvah atones for all sins.  [Laws of Teshuva 1:2-3]