throw and throws

In short: throw raises an exception. throws appears in Java on a method and announces which checked exceptions it may pass on to the caller.

In more detail: Whoever writes throw new IllegalArgumentException("age < 0") aborts the method at that point. The caller must catch it or pass it on. In Python it is raise, in JavaScript also throw, in C# throw.

In detail

void setAge(int age) throws IOException {
    if (age < 0) throw new IllegalArgumentException("age < 0");
    ...
}

• throw: statement that throws an exception object.
• throws: part of the method signature, mandatory only for checked exceptions.
• Custom exceptions: inherit from Exception (checked) or RuntimeException (unchecked).
• Rethrowing: in a catch you can pass on with throw e; or wrap the cause (new MyException("…", e)).
• Python: raise ValueError("…"), a bare raise inside except rethrows.

See also: try/catch/finally, IllegalArgumentException