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.
It can be quite difficult to identify and clean macro viruses due to the way in which they work.  Macro viruses can be set to run automatically when a document is opened, which is quite different to other types of viruses.  This is the reason why users are advised not to open attachments to emails which are not expected.  Most macro viruses would be picked up by modern antivirus software, however this does not reduce the need for caution.

What is a Macro?

A macro is a combination of commands and actions which help to automate some tasks which you would have to perform manually - in some ways these are very short, simple and basic program.  A macro virus always has to be run, or executed, from within something which understands the commands it issues - hence why they are usually found embedded within Office documents.  It is not always obvious that a document has an embedded macro virus, usually the first sign that something is wrong can be your antivirus warning you.

Transmission

Macro viruses can be transferred in similar ways to normal viruses, but are very difficult to detect.  Sometimes, for example, if two macro viruses are running simultaneously, the commands are "combined" which may not be recognised as a virus.

Macros can be (and are) often used legitimately in normal, uninfected documents.  Macros can be set to run automatically when the document is opened or closed, after a particular time period, or when the computer is idle for a particular period of time (hence avoiding detection as the user is probably not at the computer).

A big problem with macro viruses is that once infected they very quickly go on to infect every file they find, including templates, and all documents created from that point forward.  In large organisations this can be catastrophic, with every document the business uses being infected with macro viruses resulting in massive problems.

Macro viruses can, in some cases, download and install software from the internet, using automated key presses to run through the installation process without any input from the user.  This is quite uncommon nowadays but was very common in the 90's.

Perhaps the most well known macro virus was the Melissa virus from 1999, whereby a document was created with the macro virus inside it, and anyone who opened that document would run the macro and hence be infected.  This particular virus would then send itself to the first 50 people in the user's contact address book, replicating at unprecedented rates.