A function of some applications that will let you undo the command that you've just carried out. For example, it can undo a paste or a delete operation .

Unix

A multi-user, multi-tasking operating system developed by AT & T Bell Laboratories to run on almost any computer, from a PC, to minicomputers and large mainframes. There are a number of graphical user interfaces available, such as Open Look, that hide the Unix command-line .

Upgrade

To improve the performance or specification of your computer by adding more RAM, a larger hard disk or another kind of improvement. Software can also be upgraded from an old version to a more recent one.

URL

Stands for uniform resource locator. Internet system used to standardise the way in which World Wide Web addresses are written.

USB Universal Serial Bus.

A serial "information path" through which the CPU communicates with devices such as scanners. USB ports are being used on newer devices instead of the older COM ports. USB devices can be used on both IBM clones and Macs.

Usenet

Usenet groups are more commonly known as newsgroups. There are thousands of groups hosted on hundreds of servers around the world, dealing with various topics. Newsreader software is required to properly download and view articles in the groups, but you can usually post an article to a group simply by emailing to it.
 

User interface         

Software which operates between the user and the operating system, often to make the latter easier to use.

User-friendly