Baltimore is still part of the USA, so you don't have to clear customs here. :)
Here's how it works:
You hand over your luggage in Paris, and they put tags on your bags for Atlanta, and BWI. You would normally be given boarding passes for your flight to Atlanta as well as the one to Baltimore. You'll also get luggage claim checks, make sure you hold on to them in case of a mishap somewhere!
When you land in Atlanta you will go through Immigration, then proceed into the baggage claim hall. You'll pick up your luggage and walk it through Customs, they may or may not take a look at it, more often than not they just let you pass.
Then you'll see a big area, easy to find since many others will be heading for it, called the "transfer desk", where employees will be waiting to take your luggage back from you. Remember, it's already "tagged" for BWI, you just hand it over to them and walk away.
Then you need to clear TSA security, again, follow everyone else and go through the standard drill. No liquids more than 3 oz. etc. (So don't buy any booze, perfume, etc unless you remember to put it in your checked bag before you hand it over to the folks in Atlanta!!)
Once through security you can find your gate for the next flight, you'll probably have to take the tram.
5 hours will fly, it can take 90 minutes sometimes to clear the above processes.
You'll arrive at BWI like any other domestic passenger, you've already cleared customs and you just go to baggage claim and get your luggage again, and off you go.