In the case of X.Y.nnnn,

X is a major release number
Y is a minor release number, which adds some functionality or fixes bugs but is essentially still X
nnnn is a build number, which increments every time work is done and does not constitute a release

In our case we just quote the X.Y elements. We tend to use teeny increments for bug fix releases (e.g. 1.01, 1.02) and larger increments for new functionality (e.g. 1.1). Whether 1.1 should be 2.0 is hard to say. It is still very recognisably the same software, but there are quite a lot of new features. Strictly speaking emplode should definitely be a 2.0 because it was rewritten from scratch.

Rob