The Glitch is the computer virus that Ice King created in "A Glitch is a Glitch." It was created so that it would consume everything in the universal source code except for Ice King and Princess Bubblegum.
While close to deletion themselves, Finn and Jake defeat the Glitch by taking off Finn's hat and gagging on his hair. The Glitch, disgusted, starts to barf out all the source code components and was thrown out of the universal source code.
It has the same voice as Ice King, as heard when he barfs.
The Glitch seems to resemble Ice King's head with six arms. However, it is composed of computer graphical glitches, making it almost unrecognizable.