Sorry if this is a noob question, but I'm new. I'm running on OS X so, if I download and install Ubuntu and I start using Ubuntu, but for some reason I want to use OSX for a while to see something, can I change to OSX and then go back to Ubuntu??
Replacing one operating system with another
YES, there is no reason why you can't install over your Ubuntu with OS X and visa versa as many times as you like.
Using more than one operating system on the same computer
Installing 2 operating systems on the same computer is called "dual-booting" (or "multi-booting"). It depends on how you do the installation (it's a slightly different process with OSX and Ubuntu than it is with Windows and Ubuntu), but here again, the answer is YES.