Forensics is the science behind investigating crimes. Studying evidence left at a crime scene is one part of forensics.
Forensics can also be called forensic science. Whenever scientific methods are applied to a criminal investigation, that's forensics. Dusting for fingerprints, measuring the depth of tire tracks, and studying the direction of a bullet's path are all aspects of forensics. The word's Latin root is forensis, "in open court," and today much of the evidence uncovered by forensics is eventually presented in open court, as part of a legal case.