For their most popular microphones, especcially their live preformance microphones Shure has three different levels of product. The Performance Gear (or PG), the SM line and the Beta line. The SM line is Shures oldest line, many of the mics in that line have been being made for 20 years or more. Stand outs in that line include the SM57, SM58, SM87 and SM81. As time went on and materials and manufacturing techniques improved it became possible to produce better quality mics. This lead to the Beta line. Standouts in this line include the Beta58 and the Beta52. In the late 90s as more people were building home studios Shure decided to try and get in on the market and came out with their Performance Gear line for the budget concious. With one or two exeptions I think everything in this line is junk.

For the most part you can tell between the three lines at a glance. Once you develop some listening skills and have heard the various mics you'll probably be able to tell the difference by sound as well. The Performance Gear typically has a green ring around the head of the mic (Green and Gear both start with G as a reminder). The Beta mics typically have a Blue Band (Beta = Blue) and the SM gear typically has no band. This is good at a glance but if you want to be sure you should at least look at the name plate on the microphone. The bands have been known to snap, and the heads are often interchangable, so it is possible to make a PG58 look like a Beta58 at glance.

Some notables from Shure's line include

The 57s (PG57, SM57, and Beta57). Typically these are used for micing of brass insturments, and sometimes for the toms and snares of a drum kit. Some people like them for vocals, but I've never been very fond of them in that application.

The 58s (PG58, SM58, and Beta58). As I said before I consider the PG stuff junk. The SM58 is a great run of the mill microphone, you'll see a lot of them all over the place. Stands up to bumps and bruises well and still sounds good. The Beta58 is about as durable and sounds a fair bit better on people with richer voices as well as being capable in my opinion of cleaner sound at higher SPLs than the SM58.

The 81s (PG81, SM81). There is not to my knowledge a Beta version of this mic. The SM81 is generally used a lot for micing woodwinds, as well as the overheads on drum kits.