customs

We always do things differently in the States;) Your baggage will be "tagged" to it's final destination. But before leaving the Customs area, you will have to collect your checked bags from the carousels, pass through Customs and then place those bags back on another carousel to be sent-on to your connecting flight.

I can't remember DC entry, but some airports don't have the second carousel, if so, there's an airline dude somewhere that might take them from you and put on a transporter wagon of some kind. Don't worry about it -- it will be obvious as you move along.

And you have to go through security again because you have had access to your bags. But you do all of this in a separate area away from domestic passengers.

It's simple enough process, as long no one grabs your bag by accident. I did have to stop someone leaving with my bag once.

The bigger thing to remember is that you go through security again, so if you bought anything (duty free liquids, say), you have the same TSA restrictions to get into and onto your domestic flight as you would if this were your first flight.

IAD is an oddball airport in this regard. Process is the same as any other US airport point of entry. However, at IAD there are actually two areas for customs. If you have a connection, you're bags are ferried to one location and if IAD is your final destination your bags are ferried there. Either way, you collect your checked bags, pass customs and either drop them off at a bag recheck desk just past customs or you exit the airport.