A
chatterbot is a program which attempts to maintain a conversation with a person. While it is true that a good understanding of a conversation is required to carry on a meaningful dialogue, most chatterbots do not attempt this. Instead they attempt to pick up cue words or phrases from the person which will allow them to use pre-prepared or pre-calculated responses which can move the conversation on in an apparently meaningful way without requiring them to know what they are talking about.
The classic early chatterbots are ELIZA, PARRY[?] and SHRDLU. More recent programs are Racter[?] and A.L.I.C.E..