Everything LaTeX numbers for you has a counter associated with it. The name of the counter is the same as the name of the environment or command that produces the number, except with no \. Below is a list of the counters used in LaTeX's standard document styles to control numbering.
part paragraph figure enumi chapter subparagraph table enumii section page footnote enumiii subsection equation mpfootnote enumiv subsubsection
As an example, to number equations in an appendix as A-1, A-2, ... , use
\renewcommand{\theequation}{A-\arabic{equation}}The second argument implies that the text produced by the renewed \theequation command will be "A-" followed by the output from the \arabic{equation} command, which is the value of the equation counter, printed as an arabic number.
Another example of manipulating the way counters are used is given in the discussion of numbering in lists. Also see Itemize Environment for a related discussion.