Mobile Broadband cards (or aircards), are small sleek devices that plug into your laptop. They allow you to access wireless broadband on cellular networks and come in 3 main forms:
(2) PC Card
USB cards plug into the USB slot of your laptop or PC. For this reason, they are usually the most compatible with wide ranges of PC’s and Mac’s.
PC Cards, though less popular now, have the ability to function as mobile broadband cards and WiFi Cards. As most laptops now have built-in wireless radios, the usefulness of this feature has greatly diminished.
ExpressCards are the next generation of PC Cards. Slimmer, smaller and compatible with Mac’s, most providers offer these.
Which kind of broadband card you get depends on your needs. They all pretty much do the same thing. The differences they have lie in nifty features they may have. For more details check out the free "Mobile Broadband Buyer’s Guide".
There’s an entire lesson dedicated to picking the right aircard.
Lots of places to get an aircard, but there are only 4 that I recommend to get an broadband card from…