SM57 is a cardioid
-pattern omnidirectional dynamic microphone
. It has a frequency range of 40Hz-15KHz. It's primary uses are for vocals, guitars, and drums.
But like it's near-identical twin, the Shure SM58 (the SM58 has a built-in pop filter and a slight midrange tone bump that make it slightly better suited for vocals, especially in live situations), a simple description cannot do it justice. The SM57 and SM58 are the most widely used studio and live microphones ever, probably by several orders of magnitude. Their straightforward perfomance, rugged construction, and relatively low cost make them the standard microphone for so many fields that most microphones are best described in comparison to the 57/58.
It is not uncommon to see 10-20 57/58s in a studio and 1 or 2 each of all other mics (one or two large diaphragm condensers for vocals, a pair of small diaphragm condensers for drums, and various other mics for specialized circumstances or tones).