Mekhilta is an Aramaic word corresponding to the Hebrew middah, meaning a “measure” or “rule”, in this case referring to certain fixed rules of scriptural exegesis used in halakhic midrash. Though there are other works known as Mekhiltot, the title “Mekhilta” by itself usually refers to a compilation of halakhic midrash on Exodus, the Mekhilta deRabbi Ishmael, excerpted below. (The Rabbi Ishmael referred to in this case is R. Ishmael ben Elisha, a contemporary of R. Akiba.)
It is unlikely that R. Ishmael is really the author of the final work, though many of his interpretations are included in the Mekhilta, and the midrash halakhah within it falls within his school of interpretation. However, the majority of the Mekhilta actually consists of midrash aggadah, which expands upon biblical narrative, or deals with ethical or spiritual question.
The following is taken from Tractate Piskhah (Aramaic for Passover), chapter 15, a commentary on Exodus 12:43-49 (from Parashat Bo). This is one of the legal sections, gives the reader a sense of how midrash halakhah works with the biblical text. It is excerpted from Mekhilta de-Rabbi Ishmael, vol. 1 (translation) by Jacob Z. Lauterbach, published by the Jewish Publication Society.
And the Lord Said unto Moses and Aaron: This is the Ordinance of the Passover (Offering. There shall no alien eat thereof) (Exodus 12:43)
There are some sections in which the general statement comes first and the particular follows, and some in which the particular comes first and the general follows: And ye shall be unto me a kingdom of priests and a holy nation” (Exodus 19:6), is a particular statement. “These are the words which thou shalt speak unto the children of Israel” (ibid.), is a general statement. “This is the statute of the Law” (Numbers 19:2), is a general statement. “That they bring thee a red heifer, faultless” (ibid.), is a particular statement. “This is the ordinance of the Passover,” is a general statement. “There shall no alien eat thereof,” is a particular statement. When a general statement is followed by a particular, it does not include more than is contained in the particular (the fourth of the thirteen rules of interpretation of R. Ishmael).