Cross references
One reason for numbering things like figures and equations is to refer the reader to them, as in “See Figure 3 for more details.”

\label
Synopsis:

\label{key}

A \label command appearing in ordinary text assigns to key the number of the current sectional unit; one appearing inside a numbered environment assigns that number to key. The assigned number can be retrieved with the \ref{key} command (see \ref).
Thus, in the example below the key sec:test holds the number of the current section and the key fig:test that of the figure. (Incidentally, labels must appear after captions in figures and tables.)

\section{section name}
\label{sec:test}
This is Section~\ref{sec:test}.
\begin{figure}
  ...
  \caption{caption text}
  \label{fig:test}
\end{figure}
See Figure~\ref{fig:test}.

A key name can consist of any sequence of letters, digits, or common punctuation characters. Upper and lowercase letters are distinguished, as usual.
Although the name can be more or less anything, a common convention is to use labels consisting of a prefix and a suffix separated by a colon or period. This helps to avoid accidentally creating two labels with the same name. Some commonly-used prefixes:
ch 	for chapters
sec 	for lower-level sectioning commands
fig 	for figures
tab 	for tables
eq 	for equations
Thus, a label for a figure would look like fig:test or fig.test.

\pageref{key}
Synopsis:

\pageref{key}

The \pageref{key} command produces the page number of the place in the text where the corresponding \label{key} command appears.

\ref{key}
Synopsis:

\ref{key}

The \ref command produces the number of the sectional unit, equation, footnote, figure, …, of the corresponding \label command (see \label). It does not produce any text, such as the word ‘Section’ or ‘Figure’, just the bare number itself.