The Midrash Halachah is a book of interpretations of the Torah, written around the same time as the Mishnah. Unlike the Mishna, which is divided into six orders, the Midrash Halachah is written as direct interpretation of the psukim, or verses, of the Torah, and is written in Toratic order.
The book interprets all of the 613 halachot of the Torah, but starts in earnest half way through the book of Shmot, or Exodus, since there are only three halachot in the book of Bereshit, or Genesis, amongst them not eating the apple on the Tree of Knowledge!
The writers of the Midrash Halachah are the same people that wrote the Mishnah. The book is useful if one wants to know rabbinic interpretations of one specific part of the Torah, however, if one wants to find information on a specific topic, the book is less useful, although this function is performed by the Mishnah.