A wifi modem is the one you plug into your home system and whoops, you have internet. Of course, with the necessary internet subscriptions and the correct network setup.
As opposed to a USB modem, which is similar to a USB memory stick but then instead of memory, holds a 3G modem. Such a USB 3G modem is what you plug into your laptop/computer and tadaa, you'll have internet accessibility in any area where a 3G connection is available.
The difference is, the Wifi modem is the one you'd usually get for static locations such as your home, your office, etc. This is the standard internet connection you have, usually with very high, if not unlimited, amounts of upload and download, the one you can use to stream movies, browse the internet, play mmorpgs, etc.
The 3G, which is for when you are mobile: on the train, in the station, waiting for the bus etc. and you need immediate internet access for some important cause, is more expensive, has a 1GB or something limit, and if you exceed that limit, an even higher bill will be coming your way.
I checked this with my friend who works at a help desk and she approved of my explanation, so part of the credit goes to her. ;)
Depending on what you want to do, I suggest you get Wifi for a better connection at home or the office, and get 3G for emergencies when you aren't in a Wifi covered area.