The amount of zombies and waves depends on difficulty and server setting. Some run 10 rounds, some run 6, etc etc.
The concept is very simple - the game throws a set number of waves at you, increasing in difficulty. You might survive, you almost definitely won't if you try to go solo Rambo. After playing a few minutes you'll see that it's very different from L4D, though. It's grittier, harder, darker, and you feel a sense of satisfaction and accomplishment that I personally find lacking from L4D.
I believe there's a demo. Give it a shot, it grows on you real quickly. As a bonus, if you like it enough to purchase it, you'll be supporting the fantastic, customer-friendly dev team that made Red Orchestra.