Autoban is a ban applied by global mods or admins using the ban button in their profile, as opposed to manually in the admin panel. It's just the name of that particular tool.

Ban durations vary, first time offenses are usually 3-5 days, sometimes 7-10 if it's excessive. Further bans depend on time since last ban, whether the violation is the same or similar, how much that user contributes, how long they've been here, etc.

There are typically two tiers for sig spammers (or activity/post count farmers). First time 7 or 14 days, second time 14/30, third probably permanent.

I don't think autoban is a temp ban.
In https://bitcointalk.org/index.php?topic=833435.msg9315006#msg9315006, theymos said all bans listed on the modlog page are permanent.