When I wrote about fillet brazing (a lugless form of brazing) earlier, some wanted to know what was the difference between that process and welding. The quick and simplified answer is like this: welding involves heating up and joining two pieces of metal to one another directly; brazing involves using another material (one with a lower melting point) to join them, sort of like a hot glue. Last week I photographed an unpainted frame made for Josh of Bike Safe Boston that shows the two methods alongside.
Here is a shot of the underside of the frame, by the bottom bracket area. On the right is a welded kickstand plate, and next to it on the left are two little braze-ons that are (I think) cable guides. Notice the pools of golden liquid (melted brass) that surround the braze-ons, but not the welded plate. Instead, the welded plate and the chainstays it is attached to are a sort of rainbow colour, from the weld pool that is formed when joining the two pieces.
Similarly, here you can see that the seat cluster joint is welded (weld pool rainbow), but the little rack attachment point on the chainstay is brazed on (brassy border).
My description is simplistic and leaves a great deal out, but it's an easy visual for a layperson to identify brazing vs welding on a "raw" or unpainted frame. I love the colourful look the juxtaposition of both techniques creates on this one - built by Ted Wojcik and designed by DBC City Bikes. More pictures here.