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Functions are defined with multiple clauses — each pattern-matches its arguments. The first matching clause runs.

-module(main).
-export([main/1, factorial/1]).

factorial(0) -> 1;
factorial(N) when N > 0 -> N * factorial(N - 1).

main(_) ->
    io:format("~w~n", [factorial(5)]).

• Clauses are separated by ; (semicolon)
• The last clause ends with . (period)
• Guards (when ...) restrict when a clause matches

This pattern matching makes Erlang functions read like specifications:

fizzbuzz(N) when N rem 15 =:= 0 -> "FizzBuzz";
fizzbuzz(N) when N rem 3 =:= 0 -> "Fizz";
fizzbuzz(N) when N rem 5 =:= 0 -> "Buzz";
fizzbuzz(N) -> integer_to_list(N).

Note =:= for exact equality (compares both value and type). == is loose. /= is not-equal.

Anonymous functions with fun ... end:

Double = fun(X) -> X * 2 end,
Double(5).            % 10