Difference Between a Wireless Router & a USB Adapter
Wireless routers and USB adapters are common pieces of network equipment, but they serve very different purposes. To fully understand their respective uses, it's best to have an elementary grounding in the basic workings of a network.
The most basic way to connect a computer to the Internet is to plug it directly into a modem, which is in turn plugged into a signal source---a phone or DSL line, for example. Connecting multiple computers to the same Internet Service Provider (ISP), however, requires a router. Several computers can connect to this router, allowing them not only to talk to each other (forming a Local Area Network, or LAN) but to share a single modem connected to an ISP. Since routers stand between a LAN and its connection to the Internet, they also have security applications.
Just like a wired router, a wireless router connects to the Internet via a modem and allows multiple computers to share the signal. The difference is that computers do not need to be connected to a wireless router using a phone cable---the router communicates with the local network via a radio-frequency signal called Wi-Fi. This allows the computers on the network to be moved around and for authorized computers not on the network to connect to the network quickly and without the need for a cable.
A wireless adapter connects to a single computer to allow that computer to communicate with wireless networks. The adapter transmits and receives data on a Wi-Fi frequency and can connect the computer to any wireless network in the area (provided the user knows the network's security key, if any). Many newer computers have wireless adapters built into them directly. On laptop computers, external wireless adapters often come in the form of laptop cards, which plug into the computer's card reader; on desktops, they may be circuit boards that must be plugged into a PCI slot within the computer such as a graphics or audio card or---more frequently---USB adapters.
USB Wireless Adapter
USB wireless adapters are generally small devices that plug into one of a computer's USB ports. (They're often called "dongles"---a generic name for any small plug-and-play piece of hardware.) The adapters are particularly popular on desktop computers, where they are much easier to install, remove and troubleshoot than PCI adapters, although their small size and lack of an external antenna may make them less reliable at picking up weaker Wi-Fi signals, and users whose computers do not have many USB ports may be loath to give one up.
How To Use All This
To set up a wireless network in your home, you will need three distinct pieces of hardware: a modem, a wireless router and a wireless adapter for each computer on the network. Routers and adapters, then, are very different. Adapters require minimal setup time (especially USB adapters, which are designed to work as soon as you plug them in), while router setup requires a good amount of technical work, especially when it comes to network security.