Here's the deal: Laptop Tethering (also known, simply, as Tethering) means "tethering" your handset to a computer via Bluetooth or USB for use as a modem. In other words, you're using your cell phone's data connection as a gateway to get your laptop online. While this can also be done with a desktop computer, it's most popular with mobile users who need to get on the Web via laptop while away from home or the office.
Tethering works with any compatible phone and data connection, but it's generally only practical with a 3G connection - EDGE is roughly equivalent to old dial-up modems, speed-wise, and thus too slow for anything more than text-based Email and Web browsing. Surfing with your laptop via a 3G connection, however, is more than fast enough for most applications, and has become popular as Sprint, Verizon Wireless, and now AT&T have expanded their nationwide EV-DO and HSDPA networks.
The catch is that the carriers want you to use a USB data card and subscribe to pricey data plans for laptop use - they figure you're going to access the Web less frequently on a BlackBerry or smartphone than you would on a laptop, so the prices of "handheld-only" data plans are generally less than the plans geared towards mobile laptop users. As such, you'll often run into "no tethering" clauses on "Unlimited" access plans, like the Helio and Sprint deals mentioned last week.
Sprint does offer a tethering option for use with compatible handsets, but it's more pricey than their handset-only data plan. The "Sprint Phone as Modem" plan offers unlimited tethering for $39.99/mo in addition to the price of a voice plan. By way of comparison, the "Sprint Power Vision PRO Pack" offers unlimited handset-based data access along with unlimited text messaging and On Demand access for $30/mo, or 25% less.
Verizon Wireless and AT&T also offer similar tethering plans at premium prices. Note also, that "Unlimited" data access via tethering is actually capped at somewhere around 5GB/month, depending on the carrier, to prevent the use of wireless connections for constant, high-traffic applications such as serving streaming media and up/downloading BitTorrent files.