An AirCard® is a high speed wireless broadband card that gives users mobile Internet access on their laptops, using their cellular data service.
The AirCard modem comes in USB, PCMCIA and ExpressCard versions and is manufactured by Sierra Wireless for different cellular service providers such as Sprint and Verizon (you'll most often find the AirCards marketed with the name of the wireless provider, e.g., Verizon AirCard 555 or Sprint Mobile Broadband Connection Card by Sierra Wireless). Although the name AirCard is a registered trademark, many people also use the word to signify other types of mobile broadband cards.
To use an aircard, you'll need a data plan with your wireless provider; mobile broadband service plans typically are around $60/month, though some wireless carriers also offer prepaid and per-day plans as well. For that price, you get on-the-go Internet access without having to find or be at a wi-fi hotspot.