A  series is a collection of related films in succession. Their relationship is not fixed, but generally share a common key word. A series has the same characters and situation but with a different story each time.  Examples on television of series include CSI and Friends. In most cases the success of the original film inspires further films to be made, like James Bond.

 

Individual sequels are relatively common, but are not always successful enough to spawn further installments. However this is not true with the Marvel Series, the Percy Jackson series, and the Die Hard series which had four sequels.  Series are often written as they develop and are renewed if the ratings are good.