I’m confused. I’ve traded corresponding Wikipedia articles, but still can’t clearly tell what is what. Is one includes the other, or they parallel? What’s one to another?
From JewishVirtualLibrary (with modifications):
A [Baraita] is [referring to] a legal or historical document written by Tannaim (scholars of the Mishnah) that was not integrated into the Mishnah... Amoraim (scholars of the Talmud) utilized the knowledge of baraitot for the sake of attaining additional information. The baraitot would elucidate an issue or question regarding Biblical texts.
The Tosefta, is a separate compilation of baraitot passages, organized in accordance with the order of the Mishnah. The Tosefta is much larger than the Mishnah, and contains interpretations of Judaic laws. These interpretations sometimes support the Mishnah, but may also contradict its teachings.
Ie the Tosefta is a compilation of baraitot.
(other compilations of baraitot: the Mekhilta, Sifra and Sifre)