Kermit the Frog is puppeteer Jim Henson's most famous Muppet creation, first introduced in 1955. He is the protagonist of many Muppet projects, most notably as the host of The Muppet Show, and has appeared in various sketches on Sesame Street, in commercials and in public service announcements over the years. Kermit was performed by Henson until his death in 1990. Since then, Kermit has been performed by Steve Whitmire. He was voiced by Frank Welker in Muppet Babies and occasionally in other animation projects.
Kermit performed the hit single "The Rainbow Connection" in 1979 for The Muppet Movie, the first feature-length film featuring Henson's Muppets. The song reached #25 on the Billboard Hot 100. Kermit's iconic look and voice have been recognizable worldwide since, and in 2006, the character was credited as the author of Before You Leap: A Frog's Eye View of Life's Greatest Lessons, which is an "autobiography" told from the perspective of the character himself.
Source: Wikipedia, "Kermit the Frog", available under the CC-BY-SA License.