In American television, a pod is a linear sequence of commercials - it's a casing that holds a row of tiny ads, like peas. Is that cute or what? Okay, maybe not.
The number of pods per show vary dramatically, but on normal commercial television there are usually three per half-hour and five per hour. Unscripted shows (most notably sporting events or shows that cover same) are subject to the sporadic movements of balls on fields and the commercial pods therefore vary in length. Broadcast television tends to have slightly longer pods than cable television, though the two are rapidly equalizing.
The interesting thing is, if you watch enough TV you'll notice that the pods in identical programs shift depending on when the shows air - if you watch a premiering episode of The Simpsons on Sunday night there's almost always a pod immediately following the title sequence, but a year later in reruns during the week, the same episode will move that pod closer to the middle. The reasoning for this essentially works this way: People who tune in to see that Simpsons premiere are more likely to watch it no matter what; it's therefore safer to hit the viewers with more ads up front because there's only a minimal chance that they'll get bored and flip the channel. At 11:30PM on a Tuesday night, however, it's an entirely different ball game - people get flighty late at night, and the networks know they'll hold more viewers by hitting them with the show proper as soon as possible. There're other factors, but that's the gist of it.
Within the trade, segments of a show are numbered and the commercial pods are assigned numbers too, while the individual commercials are lettered - the second commercial in the third pod of some show or another would therefore be assigned the designation 3B.