Others have explained what a season is. I have to say it is changing.
1st run syndication. Shows not picked up by Fox, CBS, NBC, ABC, or CW nation wide but by local stations (Babylon 5 might be the best known outside the US)
Basic cable creating their own or picking up shows. (Burn Notice, Rizzoli & Isles, Covert Affairs) These tend to have split seasons - running 11 weeks from Season Premier to midseason cliff Hanger then having a .5 season another 11 weeks that picks up from the midseason cliff hanger to end of season. Often available on HuluPlus (fee), or for purchase on Amazon Instant Video 24 hours after the broadcast. The whole purchase on Amazon thing has had some problems because of the stop and start nature of the seasons being divided in 2. Sometimes my pass starts back up, sometimes I have to go in and by season x.5, sometimes I have to "repurchase" the whole season to restart it. I'm not charged for the Epi's I've purchased, but I always go through customer service on that one just in case.
Premium Cable like HBO and Showtime (Sopranos, Game of Thrones) I don't have cable and can't buy them until the season is over sometimes it is a couple of years. I think they run complete seasons with new epi each week for the duration of their season.
Both basic and premium cable do not have FCC rules because they aren't using the public airwaves so you get language, sex, and violence that you don't see from Broadcast Networks (Honestly I think this is the reason they are making noise about taking all their "original" programming to their cable channels if Aereo Wins their court case before the Supreme Court.
Aereo -- Instead of trying to have an antenna at my house, where I can't get CBS. I rent one and a DVR from Aereo. I can record 2 shows at a time. All shows I watch are delayed - live for about 3 seconds. Broadcasters are claiming this is illegal rebroadcast. Aereo says it is simply leasing the antennas that are strategically place to get the best reception.
Netflix/Amazon Original programming
Amazon has been doing weekly releases generally so to build "watercooler" buzz
Netflix generally dumps the whole season so you can binge watch.
One great thing about both Netflix and Amazon is they order whole seasons, and so for post the whole thing even if the ratings aren't great. Broadcast networks will often cancel something - and even if the whole season has already been filmed it isn't shown, and they don't post it for the fans to finish the season. Or in the case of Killer Women which was a limited run season - they skip 2 episodes and post the final episode. Which was very confusing - because the in the skipped episodes the main character told her family that her high placed political ex had beat her. I don't get why it would be skin off their backs to just post the whole series to Hulu - they made them.