On a Mac keyboard, what does the symbol on the ↩ key mean?
Here's the difference:
Both behave differently but are accessible through the same physical key. Historically, the return key would map to a carriage return, whereas the enter key would map to a newline. Not all applications have to keep this behavior though.
So, on a small Apple Mac keyboard, the same key can perform different functions, although by default the key sends a return—I believe you have to press fn to have it function as an enter key.
On a large Mac keyboard, there are separate keys for both, actually (as can be seen in the image below). The same applies to Windows keyboards, which have the enter key on the numpad.