Seasons represent a batch of rounds and are run over a number of months. Currently there is no set length for a season. Bots are automatically deactivated at the start of a season. This is done to avoid potential bugs with new maps so the author can first verify that their bot works before entering it into the season.
Each season will be announced in advanced and ended with a tournament.
Rounds represent a batch of games in which all bots have to play against every other bot on the ladder (Round-Robin). If the match queue is empty or the remaining matches in the existing round only contain bots who are already busy playing other matches, a fresh round is generated containing all active bots.
Ladder ELO uses an implementation as described here: http://satirist.org/ai/starcraft/blog/archives/117-Elo-ratings-are-easy-to-calculate.html
Bots start with an ELO of 1600. AI Arena use an ELO K value of 16.
The micro ladder is a new ladder coming soon that will focus on micro wars between bots. Custom maps will be made to serve as the micro arenas. More info to come soon...