Whats the difference between MouseButton1Down and MouseButton1Click?

I get those two mixed up..

There really is no big difference between the two. You can use either one and your GUI will most likely work perfectly fine with each.

The only difference is that MouseButton1Click will fire each time you make a full click - down and up. MouseButton1Down, however, will fire each time your left mouse button is down; it need not go up for the event to execute.