An HSDPA USB modem is a device used to access a particularly fast form of mobile broadband. It runs on the HSDPA system, which offers faster speeds than the commonly used 3G systems, though not as fast as 4G systems such as WiMax. It's also possible to use an HSDPA USB modem with a home computer as a replacement for fixed-line broadband.
HSDPA stands for High-Speed Downlink Packet Access. It is used to transmit data over the mobile telephone network at high speeds. HSDPA is faster than the systems currently used for most mobile internet services such as EDGE, which is commonly used for the iPhone. It isn't as fast as the "next generation services" which come under the 4G classification. For this reason, HSDPA is often referred to as 3G+ or even 3.5G.
In the case of a smartphone, high-speed Internet signals can be picked up through the phone's antenna and then processed by the phone. With a portable computer such as a laptop or netbook, the computer won't usually be able to pick up the signal itself; most computers are only equipped to pick up mobile signals through Wi-Fi or possible Bluetooth®. The solution is a USB modem, which resembles a memory stick and plugs into a USB socket. The USB modem acts as its own antenna to pick up the signal, then processes it into data the computer can use.
As with any mobile broadband service, using an HSDPA USB modem can be expensive. Most service providers impose monthly download limits which are much lower than with fixed-line services. Many users will find these limits cover their needs, but those who exceed them may have to pay extremely steep penalty or "overage" charges.
Theoretically an HSDPA USB modem can be used to provide Internet connections to a home computer where there is a good HSDPA signal. This would remove the need for a fixed line connection. Whether this makes financial sense varies from case to case, though it can work well for people who have good HSDPA coverage but either don't have a good quality fixed-line broadband service in their area or where this service is high-priced. Rather than using an HSDPA USB modem, however, most people who do use HSDPA to get their home broadband will instead use a dedicated device which plugs into a wall power socket and also includes a socket for connecting a traditional telephone.