A \label command appearing in ordinary text assigns to the
key the
number of the current sectional unit; one appearing inside a numbered
environment assigns that number to the key.
A key can consist of any sequence of letters, digits, or punctuation
characters. Upper- and lowercase letters are different.
The number assigned to the key can be accessed by use of the
\ref command. For example,
\begin{equation} ....
....
\label{eq:no1}
\end{equation}
...
One can see from Eq.~(\ref{eq:no1}) that ...
In a table or figure
environment, the \label command should follow the
\caption command (or be part of its
argument) since it is the \caption command which generates the
number.