What is difference between os.getuid() and os.geteuid()?

The documentation for os.getuid() says:

Return the current process’s user id.

And of os.geteuid() says:

Return the current process’s effective user id.

So what is the difference between user id and effective user id?

For me both works same (on both 2.x and 3.x). I am using it to check if script is being run as root.