Bio:
When Nick Fury learned that the Skrulls were planning to invade Earth, he decided to form a new team of agents, composed entirely of "caterpillars" -- "Super-powered potential agents that no one knows about - assets that haven't yet had the opportunity to be exposed or exploited." Presumably this would allow him to create a team that he could trust completely and was immune to Skrull infiltration. The new team made itself known to the world when they appeared in New York to aid the Initiative and Young Avengers in repelling the Skrull Army.
Note: This team is not known as the Secret Warriors. The title of the book that Nick Fury's caterpillars are featured in, Secret Warriors, is a reference to all the warriors featured, including the Caterpillars, Dum Dum Dugan's Howling Commandos P.M.C., HYDRA, and others.