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In Prolog you declare facts — things known to be true:

:- initialization(main).

parent(tom, bob).
parent(tom, liz).
parent(bob, ann).
parent(bob, pat).

main :-
    parent(tom, X),
    write(X), nl, fail.
main :- true.

When you query parent(tom, X), Prolog finds all bindings where the fact matches: X = bob, X = liz. The , fail forces backtracking to find every answer; the second main clause catches the final "no more answers" state.

Compound terms group data: point(3, 4), book(title('Foundation'), author('Asimov')). The same atom can describe a relation (when used as predicate) or a structure (when used as a term).

Operators are infix functions: X is 2 + 3 evaluates and binds X to 5. =:= is numerical equality. = is unification (very different — see next lesson).

Reading stdin: read_string(user_input, _, _, _, Line) (SWI-Prolog) or build your own reader. GNU Prolog uses read_atom/1 for an atom, or get_char/1 for a character.