A computer worm is a computer program which self-replicates using a network or internet connection to send copies of itself to other computers, without any user intevention.  It does not need to attach itself to an existing program - as a virus does - and hence can, and does, cause havok on networks by using massive amounts of bandwidth rather than actually damaging files.

Most macro viruses are written in languages used by Microsoft Office applications, such as Word and Excel.  These viruses are spread inside infected documents, and could also spread onto Macintosh computers as there are some versions of Microsoft Office for Mac OS.  These viruses don't tend to spread by sending out infected emails, however a small number do.

A computer virus is a small program which is designed to spread from one computer to another (hence virus) and to interfere with computer operation.  This can range from ejecting your CD drive to more severe consequences such as deleting or corrupting data, using your email application to spread itself and hence infect other computers, or even to erase everything on your hard drive.

That moment of panic ...things start happening that you can't explain ... aliens have invaded your pc?  More likely you have a virus or spyware on your computer!  The sooner you take action when you think there is something "weird" going on, the higher probability that you will not sustain major data loss or other nasties such as identity theft.