[Tex/LaTex] What does ampersand equals mean

Question

On the wikipedia page: http://en.wikipedia.org/wiki/LaTeX,
the first example is

\begin{align}
E &= mc^2                              \\
m &= \frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}
\end{align}

May I ask what does the &=do? Why is the ampersand needed?

Answer 1

It means that the equations will be aligned at this point, just before the = sign. Placing the & before the = (or other operator) ensures correct spacing around the operator.

Here you have an example (ignore the actual mathemetics, I've just randomly added some numbers for displaying propuses)

\begin{align}
E*3*2*3 &= mc^2                              \\
m &= \frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}
\end{align}

and without the &:

\begin{align}
E*3*2*3 = mc^2                              \\
m = \frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}
\end{align}