Could someone give a brief overview of how the government of the Roman Republic was set up?

I did a Web search and I am having trouble making sense of the results. This should not be rocket science.

Whatever flaws the government had, it was an ambitious attempt to incorporate conquered territories into the body politic, and it lasted as long as the empire that followed it.

I gather that there were two classes, patricians and plebians. These were more than descriptive terms. There was no social mobility. You remained in whatever class you were born into. How was this maintained? Were the patricians the only ones allowed to own land?

What was the arrangement of the legislators and consuls and how were they chosen?