Teen Girl Squad was founded as a response to Strong Bad Email: "comic" when the fan asked Strong Bad to make a comic about her four friends. Now, this should make a total of 5 girls, but Strong Bad still made a comic about 4 girls. SB even does all of their voices in the comic, and the narration, which is a little bit wierd.
Teen Girl Squad is made famous by several interesting devices. The aggressively simple line drawing style is en vogue of course. Every episode shows the girls' attempts to become more popular with the boys, and/or to look "SO GOOD!" Every episode involves many scenes of unexpected violence, and nouns unexpectedly mangled into verbs and adjectives. No character or noun is truely safe. Strong Bad drives the point home that any noun can become a grizzly adjective by adding 'd to the end of it.
In every episode, an arbitrary number of the girls get injured, killed, and/or embarrased in front of their friends. Every episode contains at least one scene where small or faraway flying things (birds far away in the sky, small hearts bubbling up from an enamoured soul, etc) get brutally killed or eaten for no sufficiently explained reason. Also, whenever several people say the same thing, Strong Bad recycles the comic baloon.