equation (environment)

From LaTeX-wiki

\begin{equation}
...
\end{equation}

Main environment for displayed formulae. Equations are numbered.

The starred version:

\begin{equation*}
...
\end{equation*}

is equivalent to the displaymath enviroment.

[edit] Referencing

The equation number can be referred to using \label/\ref (or \eqref). For example:

\begin{equation}\label{eq:my_first_equation}
...
\end{equation}
 
Equation \ref{eq:my_first_equation} shows...

Note that the label is never actually shown in the document; eq:my_first_equation will be referring to an auto-generated number.

The eq: prefix in the label is not mandatory, but is considered good practice, to keep your labels in order.