The word 'tosefta' means 'supplement'. The Tosefta is a Halakhic work which corresponds in structure almost exactly to the Mishna, with the same divisions for sedarim (orders) and masekhot (tractates). It is mainly written is Mishnaic Hebrew, with a few Aramaic sentences.
Much of the tosefta is currently regarded as being written shortly after the Mishna was redacted, and seemed to act as a supplement to it. The text of most of Tosefta agrees nearly verbatim with the Mishna, and often varies only slightly. The Tosefta offers authors names for laws that are anonymous in the Mishna; It also augments the Mishna with additional glosses and discussions.
It functions like a commentary on unquoted Mishnaic material; It offers additional haggadic and midrashic material, and it sometimes contradicts the Mishna in deciding Halakha, or in declaring in whose name a law was given.