Help to understand preforce tool, depot and workspace

Hi ( i saw youtube and preforce tutorials, i cant understand this)
What is the difference between workspace and depot ? why it is creating a copy of the project even if i set the workspace to the project itself? can i cancel this copy ? (so i have one project on my pc and one on the server, not 2 on my pc and 1 on the server as the default behavior as it is right now).

What is the difference between Submit and checkout ?

thanks : )