"One could take a look in the manual
out of sheer desperation." (based on
Herbert Voß)
Googling should have told you that align and align* are part of the amsmath package. Page 3 of its documentation reveals:
The amsmath package provides a number
of additional displayed equation
structures beyond the ones provided in
basic LaTeX. The augmented set
includes:
[...] align align* [...]
Except for split, each environment has
both starred and unstarred forms,
where the unstarred forms have
automatic numbering using LaTeX’s
equation counter.
You could also simply try out what the difference might be:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
a_1& =b_1+c_1\\
a_2& =b_2+c_2-d_2+e_2
\end{align}
\begin{align*}
a_1& =b_1+c_1\\
a_2& =b_2+c_2-d_2+e_2
\end{align*}
\end{document}