I'd also suggest a look at the Granddaddy of it all, the history of Reynolds and 531: http://web.archive.org/web/199801091...y/history.html
But you know, I gathered all I know from a multitude of sources, and I'm still learning more every day. There doesn't seem to be one encyclopedia page that sets it all out, complete and precise.
But to add a few things that Cuda2k 's OP, there's an even lower starting point than Hi-Ten (High Carbon steel) and that's regular mild steel AKA "gas pipe", it's even weaker and therefore has to be heavier to build up the toy and dept. store bikes that are made from it.
In a nutshell: you have steel alloyed with minute amounts of other metals such as chromium, manganese, molydenum...531 reportedly was a description of the percentages of alloying metals: 5%, 3%, 1% added to the steel.
Then you have different manufacturing techniques: Reynolds patented the method of cold drawing a tube through a die with an internal mandrel that was adjusted to make a butted, thicker internal diameter at both ends, leaving it thin in the center and a consistant external diameter...amazing when you think of it!
You could also draw it hot, draw it in a simple consistant diameter (plain guage, not butted), or roll up a flat strip and weld a seam to make a tube (rolled and seamed) either making a simple or a butted tube.
Innovations came along like internal grooves or splines added to steerer tubes, and then to the other tubes (specifically Columbus SL/SP and then SLX/SPX), as well as multiple-profile butting (specific to several Japanese makers like Ishiwata, Miyata, and Fuji) called triple and quad butting.
Then you have different dimensions: metric size tubes, standard tubeset, oversized...then adjustments to the guage (like 0.7-0.9) describing the thickness of the main tube to the butted section of a DB (double-butted) tube. The guage can be adjusted while keeping the external diameters standard, to produce lighter or heavier tubesets, appropriate for different bikes, different size frames, different user requirements and different construction techniques.
And that last item (construction) is what resulted in the more recent innovations: hardening of thin guage steel alloys to permit lightest weights and greatest FINISHED strength when TIG welded. Alloys have been further fine-tuned to create the ultimate specialized product tailored to the manufactuer and his product...but since there's less demand for steel bikes (or just more profit in aluminum), there are fewer steel alloy tubesets available to the builder in 2007 then back in 1995. Reynolds and Dedacchai are still pulling steel tubes, Columbus might be, but many others have dropped out.
There's a quick overview, as you see there's a lot to learn. I wish Sheldon would write up the definitive article on his site!