What is a Starcraft 2 "season"?
In the SC2 online help, it's mentioned that leagues are played in seasons, and "the ladders will be wiped, allowing everyone to have a fresh start and begin anew."
Are there any details on this? How often will this occur? Will everyone have to be placed into leagues again?
"Season" in this case, refers to the same thing as a "Baseball Season" or "Football Season". Blizzard does this on most (all?) Battle.net games including Warcraft 3 and Diablo II.
"Wipe the ladders" is quite literal -- everyone, from the 'leetest' of the 'leet' to the worst of the worst gets a rating reset and must partake in their placement matches again. (But hey, at least that's another chance at the Hot Shot feat of strength!)