From the docs:
When browsing the frontpage or a community, you can choose between the following sort types for posts:
Active (default): Calculates a rank based on the score and time of the latest comment, with decay over time.
Hot: Like active, but uses time when the post was published.
New: Shows most recent posts first.
Old: Shows oldest posts first. Most Comments: Shows posts with highest number of comments first. New Comments: Bumps posts to the top when they receive a new reply analogous to the sorting of traditional forums.
Top Day: Highest scoring posts during the last 24 hours.
Top Week: Highest scoring posts during the last 7 days.
Top Month: Highest scoring posts during the last 30 days.
Top Year: Highest scoring posts during the last 12 months.
Top All Time: Highest scoring posts during all time.