Data cards are cellular devices that either plug in to your card slot on your laptop, or more lately, via USB, although some have their own little wireless networks. While they are technically cell phones with a number, they are dedicated to data only. Works a lot like after you tether, without the phone.
BTW, you might want to check the fine print your contract on Sprint. I was gonna set up my BB to tether as a backup plan, but even on the unlimited everything including the kitchen sink plan, tethering still was not covered.