A search warrant describes the particular location to be searched and the things to be seized:
no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized
In Groh v. Ramirez 540 U.S. 551 (2004), a search warrant failed to do this, and it was held invalid:
The warrant was plainly invalid. It did not meet the Fourth Amendment’s unambiguous requirement that a warrant “particularly describ[e] … the persons or things to be seized.” The fact that the application adequately described those things does not save the warrant; Fourth Amendment interests are not necessarily vindicated when another document says something about the objects of the search, but that document’s contents are neither known to the person whose home is being searched nor available for her inspection.
However, if while executing the search warrant, the government comes across evidence "in plain view", it may be seized and is admissible in court.
Coolidge v. New Hampshire 403 U.S. 443 (1971):
Under certain circumstances, the police may, without a warrant seize, evidence in "plain view," though not for that reason alone, and only when the discovery of the evidence is inadvertent.
An example of the applicability of the "plain view" doctrine is the situation in which the police have a warrant to search a given area for specified objects, and, in the course of the search, come across some other article of incriminating character.