As far as I have seen, permenant bans have only been issued to people who are repeat offenders, and people who recently registered who break some serious rules, like posting porn or trash talking the mods by making rant posts about them, spammers... Basically people who have nothing to contribute to the forum as a whole tend to get the boot.
Temporary bans are usually issued by people who just break the rules. They are also given out to people overstep the boundries or do purposely stupid things... like quoting the same thing 10 or 20 times.
As a rule of thumb, if you have any doubt about what you are posting, either don't post it or PM AnandtechModerator and ask them if it is okay.