Answer:
   
   Medrashim are compilations of tannaitic material organized as a
   commentary on the Torah. There are two sorts: medrashei halakhah
   (halachic medrashim) record tannaitic discussion of the halachos
   raised by the verses, and medrashei aggadah are the discussions of
   everything else: theology, philosophy, ethics, the human condition,
   etc. (Medrashei aggadah are usually composed using stories and
   metaphore, and gave the word "medrash" a second meaning of stories
   that embellish those in the text or are about figures known to the
   authors.)
   
   There is also a Mekhilta deRabbi Shim'on bar Yochai, but that is
   rarely referred to, and therefore people would call it by the full
   name. It's not "the Mekhilta". This other Mekhilta is sometimes called
   Mekhilta deVei Rabbi Aqiva (the Mekhilta of the house/school of Rabbi
   Aqiva) as the identification of the school of the author is more sure
   than the who in that school actually wrote it.
   
   This split is quite relevent as Rabbi Yishma'el and Rabbi Aqiva had
   very different theories about how the rules of derashah, of
   associating halakhos with the text by hermeneutic rules, work.
   Therefore their medrashei halakhah reflect such differences.
   
   The two series of medrashei halakhah are:
     * R' Aqiva's school:
          + Mekhilta deRabbi Shim'on bar Yochai (on Exodus)
          + Sifra (on Leviticus: a/k/a Sifra deVei Rav, Toras Kohanim
          + Sifrei Zuta ("the small sifrei" on Numbers)
          + Sifrei (Deuteronomy)
     * R' Yishma'el's school:
          + Mekhilta (a/k/a Mekhilta deRabbi Yeshima'el, on Exodus)
          + Sifrei (Numbers)
          + Mekhilta Devarim (the Mekhilta on Deuteronomy)
       
   The traditional publication of the medrashei halakhah includes four
   books, mixing the two schools: Mekhilta, Sifra, Sifrei (on Numbers)
   and Sifrei (on Deuteronomy). In fact, the two Sifrei's often get
   published as a single volume, despite the differnce in style that
   makes their different origin obvious.
   
   A more complete publication would have all seven books, traditionally
   published in the order: Mekhilta, Mehilta deR' Shim'on bar Yochai,
   Sifra, Sifrei (Numbers), Sifrei Zuta, Sifrei (Deut), Mekhilta Devarim.
   
   The word "mekhilta" is Aramaic, and means "measure" or "rule". The
   words "sifra" and "sifrei" are conjugations of the root /spr/, meaning
   "book" or "writing a book". Sometimes the word "sifrei" is used to
   refer to all 4 books.
   
   After Rabbi Yehudah haNasi compiled the Mishnah, organizing halakhah
   by topic rather than verse, the notion of composing medrashei halakhah
   fell out of use.