Airport express question

Hey guys.
I'm thinking about getting airport express as I'm sick of having to cart my printer over to my iMac everytime I want to print.
Anyway I want to know how the express works as a router.
I already have a wireless modem/router so I have no troubles there.
If I set up a wireless network does the express always have to be plugged into the modem which is not where the printer is.
Sorry if this is confusing but I've got a) iMac in room, b)printer in study and c) modem/router in living room.
I would assume I would have to have it plugged in to the printer at all times or does it always need to connect to modem?

Cheers

Airport Express can act as an "extender" to your current wireless network i.e. it can connect itself wirelessly to your existing and network and you could use the Ethernet port to put a device onto your network.
You could put have AE in the study by the printer and it would supply an Ethernet connection for your printer to plug into (it is talking to themain modem/router wirelessly).

I am using my Airport Express to extend my wireless network into our Family Room and it works great.
You can read the instructions on how to set it up as an extender on the Apple Support Site.

I hope I have not confused you further.
DavidH