What does the version number of gcc mean ?

["d hongqian" <dhongqian@xxxxxxxxx>]

Hello:
    I want to know  the version of gcc 's  mean. For example: the gcc
3.4.6, what's the difference between the 3.4.6 and 3.4.5.
and what's difference between 3.3.0 and 3.2.0 and  what's the
difference between 3.0.0 and 4.0.0.
    If I want to update my gcc, how many things should also be update
so that the system could be stable.

Thanks !