There are three possible sort options for answers, the default being sorting by votes, and you can always choose a different sort by clicking one of the links to the right of the answers amount title:
Once a sort is selected, it is "remembered" on the same site and associated with your account. This means that whenever you visit the site again, even from different browser, the sort you selected will be used as long as you're logged in.
Each sort option has a tooltip explaining what it is, I'll give more details below.
That's the default and most common sort order, placing the answers with the highest score (upvotes minus downvotes) first, with the exception of an accepted answer being on top regardless of its score, unless it was posted by the question author. Deleted answers (for users with the Access To Moderator Tools privilege) will always appear last in this sort. Answers with the same score are sorted randomly, every time the page is loaded. This is useful because a higher score means a more helpful answer, so naturally people having the same question would like to see the most helpful answers first.
As the tooltip says, this sort puts the answers in the same order they were posted, meaning sorting by time of creation, descending. The first answer posted will appear first, the last answer posted will appear last, regardless of score or edits (and regardless of deleted status for 10k users).
This sorts the answers by the time of last activity of each answer, descending. Activity for answers means editing, or moderator actions like locking/unlocking. This means that in this sort option, answers that were recently edited will appear first, regardless of score.
Let's have an example too. Suppose a question got three answers:
- Answer A posted a year ago, having a score of 10, edited 10 months ago.
- Answer B posted two months ago, having a score of 15, edited today.
- Answer C posted yesterday, with a score of 1, not edited.
This is their order of appearance using the different sort options:
- Votes: B, A, C
- Oldest: A, B, C
- Active: B, C, A
Also worth mentioning that even though an answer's score is "live" (i.e. changing without reloading the page), it does not affect the location of the answer, i.e. the sort will be applied only when you reload the page.