A tricorder was an all-purpose hand-held scanner, capable of detecting and locating anything from viruses in a body to a subspace anomaly.
There were three types of tricorders each designed for a specific purpose, the science tricorder, the medical tricorder, and the security tricorder. (FASA RPG module: Star Trek: The Next Generation Officer's Manual)
Tricorders could pick up communications which could be listened to via an earpiece attachment. (DS9 novel: Devil in the Sky)
Tricorders could be used to mark a target for an airstrike by ship or shuttle. If wounded by an energy weapon, they could also provide a quick healing boost or restore personal shielding to full strength. (TOS video game: Star Trek)