A serialized program is one in which each episode picks up where the previous one left off. They are characterized by cliffhangers, extended story arcs, and often complicated character histories. Serialized shows encourage a loyal viewership, but they can also frustrate casual viewers, and make it difficult to start watching a show in the middle of its run.
Soap operas are the most common serialized shows, but other formats can be serialized as well. Action/adventure shows (Lost, Alias, and 24), science fiction (Babylon 5, Battlestar Galactica), and even some comedies (Soap and Arrested Development) have elements of serialization.