As long as you watch from the first hour of the new season you'll be fine. I jumped in on the third season halfway through and still got hooked. There are lots of episode guides out there if you want to look for previous relationships as you get more familiar with the characters. Each season is a self-contained story, though frequently ending on a cliff-hanger note leaving you wanting more (such as the last one leaving Jack on a slow boat to China).
As the show's name implies, each season is a single day, consisting of 24 episodes. It runs in "real time" for that day. Certain liberties are taken, such as cell phones that are never low on battery or signal.
Good luck!
...Jack has the strongest bladder in the universe.
I jumped right in tonight, and you were right. I didn't need to know any backstory. They filled in what I didn't already know. Pretty engaging stuff.
Unfortunately, I have terrible Fox TV reception in our mountain valley, but I refuse to get cable. I had to watch the whole thing with the sound down because the static was so loud.