USB stands for Universal Serial Bus. This is a serial bus standard for interface devices. It was originally created for personal computers, but since its creation USB has become very common on hand held devices like video game consoles, portable memory devices, Personal Digital Assistants, and portable media players.
USB is a major component in the legacy-free PC, and it was devised to help get rid of all parallel and serial ports on the personal computers because these ports are not standardized and had a requirement for numerous device drivers that had to be developed and maintained.
A USB system has a design that is asymmetrical, and consists of a host controller along with multiple peripheral devices that are daisy-chained. Any additional USB hubs can be included within the daisy chain, which allows for branching into a tree structure, subject to a limit of 5 levels of branching per controller.
No more than 127 devices, which includes the bus devices, can be connected to one single host controller. The modern computers available today often have several host controllers, which allows for a very large number of USB devices to be connected.
USB cables do not have to be terminated. The ability of USB to daisy chain devices led to an early prediction that each USB device would include a downstream port to allow for long chains of devices. Many of the first computers that were shipped with USB ports only had two USB ports.
Because of technical and economical reasons, daisy-chaining never became very widespread. Desktop computers today are usually available with six USB ports to reduce any need for USB hubs, generally with half of the available ports in back and half on the front panel for easy temporary connections of any portable devices.
In the market today, USB has become widely available on mobile computers. Laptop and notebook computers are available today with at least one, and possibly multiple, USB ports. Apple, Dell, and almost all the other computer manufacturers have several models of mobile computer that has one or more USB ports that are reasonably priced.
Another type of mobile computer today is the cell phone computer with Internet capabilities. This type of computer has a certain type of USB port that is called a mini USB port. This port allows for the connection between the cell phone and a computer or other device. Personal Digital Assistants also have mini USP ports for connection to the computer.
To answer the question does USB availability apply to mobile computers as well as desktop computers, the answer is yes. Almost all mobile computers that are available today come with at least one USB port, some with multiple ports.