A cell-site simulator poses as a cellphone tower and tricks a targeted cellphone into connecting with it instead of with a tower. Law enforcement officers might use a cell-site simulator to locate and track suspects associated with a particular cellphone. This is different from asking the phone company to disclose either real-time or historical cellphone data; this is direct government surveillance.
And that is why the D.C. Court of Appeals held last week that the use of a cell-site simulator to locate or track a suspect is a search for Fourth Amendment purposes, and ordinarily requires a warrant. The majority, concurring, and dissenting opinions of Jones v. United States are all well worth reading for a thorough examination of this issue, how it differs from other cellphone issues (such as the one before SCOTUS this term), and what other courts have held with respect to this and related issues.