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: 
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}

```

![enter image description here](https://i.stack.imgur.com/AR6nY.png)

and without the `&`:

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

```

![enter image description here](https://i.stack.imgur.com/oTXtR.png)
They are not a combined symbol `&` is the alignment point in `align` just as it is in `tabular` It marks the division of the row into cells which are then stacked vertically. The second cell in each of those rows happens to start with `=` so Tex will typeset the rows with the `=` one directly above the other,