Hi Ricky,
The client will only run when the screensaver is active. It does not run in the background. To have it always running in the background you will have to use the command-line client, aka ASCII client.
When you stop the screensaver, I believe it will buffer the structures created or attempt to upload them at that point. I don't recall off-hand as the screensaver does not get along with non-Intel processors which precludes me since I run AMD systems.
Definitely it will try to send in the structures if it reaches the number of structures specified by you to upload at. Assuming you have auto-dial, your computer should try to make a connection and upload the structures stored.
If not, or if so and the connection is not made successfully, the structures will be buffered (stored) until it can make a connection with the Distributed Folding website, which means you will need to launch the screensaver when the connection is up.
You might try out the ASCII (command-line) client. Its faster than its screensaver counterpart and most of the people run it place of the screensaver. Using dfGUI (http://www.gilchrist.ca/jeff/dfGUI/ ) you can easily setup and run the command line client.
Whatever the case, enjoy, and feel free to post again with more questions. This is definitely the place for it.
RuneStar½