Overloading is when two or more methods have the same name but different signature (different argument types, different number of arguments). Overloading is resolved statically, depending only on the static types of the arguments. (The interaction of overloading and overriding, in Java, for example, makes the story a little more complicated). Overloading resolves statically to a particular method signature. Then at runtime, dynamic dispatch will select the most appropriate method with that signature.
Multimethods occur when two or more methods have the same name, the same number of arguments, and overlapping type signatures. Multimethods are resolved dynamically, the most suitable method is chosen depending on the runtime types of the arguments. All arguments are used to determine which method body to run. This differs from traditional single dispatch in Java, where only the first argument (the target) of the method is used to determine which method body is run.