I've seen people use the SM7B for recording singing, but I thought it was a broadcast microphone?

While it's true that the SM7B is designed with radio broadcast in mind, it's very popular for use in the studio. In fact, the SM7 it improves on was used for recording vocals on Michael Jackson's Thriller and Bad albums, as well as Metallica's black album.

Is the SM57 designed only to be used on guitar cabs?

The SM57 is the microphone most often used with guitar cabs on stage and in the studio, but this isn't its only use. It's common to see an SM57 used on snare drums, acoustic guitars, violins, and even vocals.

What's the difference between the Shure SM58 and Beta 58A?

The SM58 is a legendary microphone that was launched in 1966. Pretty much every singer has used an SM58 at some point! The Beta 58A was launched in 1996 and takes what's great about the SM58 and expands the frequency response. It also reduces the proximity effect that the SM58 is known for and gives out a slightly "hotter" signal. The Beta 58A is generally considered a more modern sounding microphone.