I have next to me Walker Evans American Photographs and Robert Frank's The Americans. These are two of the more important monographs in American photography, and for good reason. They've got some good pictures in them.
Robert Frank is often cited as one of the seminal "street photographers", and was a student of Walker Evans. I own the Evans book and am generally pretty familiar with his work. Robert Frank, of course I know some of his pictures because they're pretty universally known (even if you don't know who shot them) but mostly I wasn't familiar with the work. Anyways. Onwards.
My first impression of The Americans was "hey, he's a beat Walker Evans!" which was probably influenced by the fact that Jack Kerouac wrote the intro, and my knowledge of both the era Frank was working in and the fact that he was a student of Evans. A beat Walker Evans is exactly what one might expect. After due consideration, I think this summary is still pretty spot on.
Both photographers have that foursquare, head-on style that so exemplifies Evans. Here is a thing. Here is a person. This is what it looked like. And yet, these are not trivial pictures. For reasons that are not always clear, most of them are pretty good. Most of them not only show us the immediate thing, but get at in some way something more fundamental, something underlying the thing. Evans liked his diagonals, either a diagonal thrust to lines, or simply interesting objects placed in opposing corners. Frank doesn't seem to like them as much. Frank has a few other quirks, preferring fragments perhaps more than Evans. Franks pictures are a lot rougher around the edges, more out of focus material, more motion blur.
In general, though, the pictures are a whole lot more alike than they are different.
Every time I though "aha! Here's something Evans never did!" I'd flip the American Photographs and find an Evans shot where did did exactly that. And vice versa, for the most part.
The substantive difference, and the one that makes Frank "street" where Evans wasn't, is that Evans prefers to show us the scene, the thing, the person, where Frank -- sometimes -- shows us the moment in time. Not always, much of The Americans is very much Evans, very much the scene, the person, the object. But here and there, we see an instant, a moment in time that was there, click, and then gone. The moment captures, or makes us believe that it captured, the event.
Evans shows us a row of politicians in fancy hats. Frank shows us a row of policiticans in fancy hats, at the instant one of them stifles a yawn.
It's an interesting evolution, one well worth looking at. Frank strikes me as someone straddling the genres, between whatever you call what Evans did (straight photography?) and "street".