The first episode of a new TV series is called the pilot. If you pitch a TV series to a network, and they're interested, they'll order a script. If they like the script, they order a pilot. You get a small budget and you get to make the first episode of your show. The pilot is reviewed internally. Sometimes they order a couple more, sometimes they order the season outright, but more often they pass.
There are a lot more pilots made than series.
I have made pilots, and I have seen friend's pilots that never made it to air, and I have seen random pilots that were re-made with notes from networks on casting etc.
Chris Fleming's looked really neat:
Frankly the only thing I am interested in seeing are canceled pilots. I would pay premium prices for a channel that was nothing but dead pilots. I only want to watch what the powers that be think I don't want to see.