I have an internship with a company that does work with VoIP, and I hear terms like "gateways" and all those VoIP terms a lot. Basically, a gateway is a router configured to connect to the VoIP world (at least in our case, we have a hosted VoIP system) whether that be a "softswitch" (basically what ThoughtPhreaker said, a bullshit marketing term to make people buy their product), a PRI, T1 line, or a POTS line. The analog equipment (like a POTS line) is connected using cards installed into the gateway, we use FXO cards to connect our POTS lines to the gateways. The remote buildings have their gateways pointed towards our call manager, from there we manage all of the Cisco VoIP phones. Oh yeah, I forgot to mention, all of our equipment is Cisco.
On another note, I find it funny how Cisco run EVERYTHING when it comes to IT! All of our phones are Cisco's, all of our cards are Cisco's, hell Cisco could easily be compared to Ma Bell back when they ruled the telephone system. Their VoIP phones are built well, but god forbid the back stands breaks on the back of it, they are a pain in the ass to take apart and put back together. I'm not kidding when I say it took me fucking 20 minutes to try to put the back on a Cisco 7940!