Brazing is much like gas welding, except that the filler material you use has a lower melting point than the metal you are joining. The most common filler is probably bronze. Basically, you heat the steel up so that it is hotter than the filler's melting point (but not hot enough to melt the steel) and then apply filler.
It is most commonly done with a plain old oxy/acetylene torch with a soft flame. You could also do it with a mapp/oxy torch.
The nice thing about it is that there is no puddle to worry about, no burning holes through anything, and the bond is stronger than the steel itself (according to two welding books I have).
Ed
