What is the Difference Between 'alias' and 'alias_method'?

Ruby has this...  thing called 'alias.'  It'll create a second name for a method (or other things), but it doesn't really fit in with the other Ruby-isms.  It's a keyword and not dynamic at all.  But there's also an 'alias_method' method that does (almost) the same thing.  Why are there two?