His name translates, roughly, as "drill tooth." He is a red squirrel found in the earliest written compilations of Norse mythology, dating to approximately the thirteenth century and based on earlier oral traditions.
Ratatoskr's job, apparently, was to climb up and down on Yggdrasil, the world tree, and carry messages back and forth between the eagle perched at the top, to the wyrm, or serpent, Nidhogg who lived beneath one of the roots of the tree.
This is how Ratatoskr is described in one translation of the poetic Edda (Icelandic collection of Norse myths):
Ratatosk is the squirrel who there shall run
On the ash-tree Yggdrasil;
From above the words of the eagle he bears,
And tells them to Nithhogg beneath.
Another Icelandic collection known as the Prose Edda also describes Ratatoskr, but in this version he seems more a villain, or at least a bit mischevious:
There is much to be told. An eagle sits at the top of the ash, and it has knowledge of many things. Between its eyes sits the hawk called Vedrfolnir [...]. The squirrel called Ratatosk [...] runs up and down the ash. He tells slanderous gossip, provoking the eagle and Nidhogg.