What is the difference between a Toshav and Sachir?
Rashi and Targum Yonasan: A Toshav is a Ger Toshav (who undertakes to observe all the Sheva Mitzvos b'nei No'ach), and a Sachir - a Nochri (an Arabi Mahul or a Giv'oni Mahul, who are Pasul even though they are not [physically] Areilim). 1
Ramban (according to the conclusion of the Gemara in Yevamos, 71a): A Toshav is a Ger who made Bris Milah but not Tevilah.
See Ramban's objection to Rashi's explanation, and the commentary of Rav Chavel.
Rashi writes that Toshav is a Ger Toshav, and Sachir is a Nochri Mahul. Why is this unlike Terumah, for which Toshav is a Nirtza (an Eved Ivri who serves until Yovel), and Sachir is an Eved Ivri who serves for six years?
Riva (51): It is reasonable to expound so about Terumah, that they are not the Kohen's property, so they may not eat Terumah. For Pesach, since he is a full Yisrael, surely he is obligated!
Rashi writes that Sachir is not needed for a Nochri, for the Torah already disqualified an Arel. He should have said that it already disqualified a Ben Nechar!
Riva: Rashi gave the simpler reason, even though the real source is from Ben Nechar. Also the Talmud sometimes gives the simpler source, even if it is not truly the source!