a little confused (well, maybe a lot) about 12 lost tribes and current Jewish "division" : is Ashkenazim Jews one of them (12 tribes)? What about Sephardi?
Apperentaly, yes, the Ashkenazi Jews are (part of) the Tribe of Judah (which is the source of the word "Jew", in Hebrew "יהודה" is the Tribe of Judah and "יהודי" is Jewish) and the Tribe of Binyamin (those two tribes were not exiled with the rest of Bnei Yisrael). And so are the Spheradic Jews.
In Addition, the Tribe of Levi was disperssed and assimilated within the other tribes and thus, when we lost contact with the Ten Lost Tribes, a portion of Levi members remained with Judah.
However: in Tragum Yonatan (Bereishis, 49, 8) he writes that the blessing to Yehuda included that all the tribes be named after him, and some say that from the Tefila in Parashat Haneseiim of the month Nissan it is implied that it is unknown. Furthermore, some claim that the Ashkenazi Jews are might in fact the decendants of the ten tribes and they "prove" that claim from the Book of Obadiah (end of chapter 1) which is the Haftara of Parashas VaYishlach. It is seems to be opinion of the Aruch and the Maharshal (on the Gemara Yevamos, page 17). And you can find more info in the book "וַאֲשֶׁר תָּבֹאנָה יַגִּידוּ".
Ashkenaz and Sfard are modern divisions based on where those groups lived during the time of the galus (exile). They have nothing to do with the original division into the twelve tribes. Consider that after the revolt against Rechav'am (son of Shlomo Hamelech) many members of the twelve tribes resettled in Yehudah because they maintained loyalty to Hashem. After the dispersion of the ten tribes, more of them fled to Yehudah. After the destruction of the two temples many lost the identity of their original tribes.
The term Sephardi means "Spanish" and results from the expulsion from Spain in 1492. That is why others (such as the Yemenite Jews) are called Eidot Hamizrach (the Eastern Communities).
Ashkenaz means Germany (though it includes Northern and western Europe) and after the expulsions of Europe they basically settled in Eastern Europe.
Other communities became mixed with these two main groups. Since the Shulchan Aruch and the Ramah's gloss became the main halachic standard, they are the two groups that are most known.