The sawtooth wave is a kind of basic waveform. It looks a little bit like the blade of a saw - therefore the name.
The sawtooth wave can be described by a linear function: y=x. It rises to one in an angle of 45 degrees then goes down to -1, again with an angle of 45 degrees - considering the x-axis. From there it goes up again to zero. So at 1 and -1 you have an angle of 90 degrees.
Its sound is harsh and clear. It is maybe the best waveform to construct other sounds in terms of subtractive analogue synthesis. One can also construct it by additive synthesis. One can build it of sinewaves (Fast Fourier Transformation).
Σsin(nx)/n is a sawtooth wave.