VESA: Video Electronic Standards Association - This industry organization formed to create various personal computer standards, including those for Super VGA video displays and the VLB bus standard.
Virtual Memory: Virtual memory is really a part of your hard disk called a swap file, dedicated as a storage area for bits of data in RAM that aren't being used much. By freeing up RAM, you're virtually increasing the amount of working memory available to you.
Virus: A virus is a piece of code usually disguised as something else that causes some unexpected and usually undesirable event. A virus is often designed so that it is automatically spread to other computer users. Viruses can be transmitted as attachments to an e-mail or files, for example.
Virus Definitions (dat's, signatures, database): Virus Definitions, sometimes called dat's, signatures or database updates are files that update your anti-malware (anti-virus, trojan and spyware scanners for example) programs. These make sure that your program(s) can recognize the latest known threats. You should always look for an auto update feature and allow your anti-malware programs to update as needed.
VPN: Virtual Private Network - A Virtual Private Network, or VPN, is a private network of computers that's at least partially connected by public phone lines. A good example would be a private office LAN that allows users to log in remotely over the Internet (an open, public system). VPNs use encryption and secure protocols like PPTP to ensure that data transmissions are not intercepted by unauthorized parties.