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 some 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.
In List Environments the format for the item labels is given by commands like \labelitemi and \labelitemii (for the first two levels of the Itemize Environment), and like \labelenumi (for the Enumerate Environment), and these may also be redefined.
There is an example of manipulating such counters in the discussion of the List Environment. See also the discussions in the Itemize Environment and in the Enumerate Environment.
You can force a change in the number produced by any of the counters. For example, if you wanted to produce a single page numbered 13, you could use
\setcounter{page}{13}
A variety of commands for manipulating counters this way are given below.