The official definition is:



A macro  in computer  science
<http://en.wikipedia.org/wiki/Computer_science> is a rule or pattern
<http://en.wikipedia.org/wiki/Pattern> that specifies how a certain
input sequence (often a sequence of characters
<http://en.wikipedia.org/wiki/Character_%28computing%29> ) should be
mapped to an output sequence (also often a sequence of characters)
according to a defined procedure. The mapping process which instantiates
a macro into a specific output sequence is known as macro expansion.



In short, a macro is a sequence of instructions carried out by the
computer. Within Excel a macro is more of a recorded sequence of
actions. You can record a macro from the front end and the computer
creates the sequence of code for you. This however is quite limited in
comparison to what a fully written program is capable of consisting, but
is sufficient for basic front end Excel tasks, especially those that are
done repeatedly within the environment.



A basic example is you can record a macro for every time you highlight
and bold the first row of any Excel sheet where you use headers, once
recorded you only need execute the macro to repeat the process.