Answer:
   
   Rabbeinu Tam (12th cent) is one of the better known Tosafists, and a
   grandson of Rashi. His real name was Rabbi Yaakov ben Meir. Yaakov
   (Jacob) was called in Genesis an "ish tam", a whole/perfect man.
   
   In halachic rulings, the Tosafists usually defended Ashkenazi norms,
   trying to find its basis in the Talmudic texts. Rabbeinu Tam was an
   exception. Two rulings of his are better known, they serve as an
   illustration. In both, Rabbeinu Tam disagrees with the opinion of his
   grandfather.
   
   The first was on placement of the mezuzah. Rashi ruled that proper
   placement is that the parchment be placed vertically. Rabbeinu Tam
   sided with a horizontal placement. Today, Ashkenazim try to fulfil
   both opinions by hanging the mezuzah on an angle. Sepharadim follow
   the opinion attributed to Rashi, which is also born out by the
   majority of archeological findings. (But they do so because it's the
   opinion of Maimonides and the Shulchan Aruch.)
   
   The better known one was about the proper order of chapters that are
   placed in tefillin. We follow the opinion of Rashi. However, many
   Chassidim also wear "Rabbeinu Tam tefillin" (as they are commonly
   called) afterwards to fulfil his opinion. Some Sephardic kabbalists do
   as well; while a rare few wear both forms simultaneously!
   
   His style of Talmud study was more typical of the Tosafists. They
   tried to understand the Talmud holistically, typically asking how one
   discussion ought to be understood given a discussion in another part
   of the Talmud.

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