Cichlids are the biggest and most diverse group of fish to be kept in the home aquarium.  Wikipedia defines a cichlid as having one specific identifying feature:  the fusion of the lower pharangeal bones into a single tooth-bearing structure.  There are other identifying features too, but in everyday life, we recognise cichlids purely from that being the name given to a whole variety of very popular breeds.  Generally speaking, when people refer to keeping cichlids, they will more often than not be referring to two distinct groups of fish:  African cichlids from the lakes in Eastern Africa, notably Lake Malawi, and a range of larger fish from South and Central America.  The two groups need totally different conditions, and should never be kept together.  Unusually for fish, almost all cichlids make fabulous parents, but with that comes their deserved reputation for aggression towards tank-mates.  Research the species you are interested in well before purchasing any.