Fundamentals
A macro is a series of commands and actions that help to automate some tasks - effectively a program but usually quite short and simple. However they are created, they need to be executed by some system which interprets the stored commands. Some macro systems are self-contained programs, but others are built into complex applications (for example word processors) to allow users to repeat sequences of commands easily, or to allow developers to tailor the application to local needs. The step which has made some applications susceptible to macro viruses was to allow macros to be stored in the very documents which are being edited or processed by the application. This makes it possible for a document to carry a macro, not obvious to the user, which will be executed automatically on opening the document.
Read more about this topic: Macro Virus