In fact, this structure will be familiar if you have studied logic. A group starts with a name followed by an open bracket. This is called the predicate. After the bracket come a sequence of names or numbers separated by commas. These are called the arguments of the predicate. Finally, there comes a closing bracket. Note that the word ``argument'' is used in a technical sense peculiar to mathematics; this sense has nothing to do with its everyday meaning.
Predicates can have any number of arguments, so groups like these are all valid:
job(mp) loves(john,mary) pub(prince_of_wales,walton_street,oxford,morrells)
If a predicate has no arguments, then you just write its name:
mp hotProlog will not allow you to have the name followed by an empty pair of brackets, though some other languages do.