Deckers are "computer hackers" who use a cyberdeck to jack into the Matrix (the worldwide information grid and computer network). By projecting their minds into the electronic world, deckers can find secrets others keep hidden away. Using programs and Expert Systems to fight off intrusion countermeasures, they can provide invaluable services during a run.
For the most part, Deckers play like Street Samurai in the real world. However, their true utility lies in the ability to enter the Matrix and provide a variety of useful services, including acquiring confidential data (typically to sell on the black market for nuyen), shutting down security systems, and otherwise aiding the rest of their shadowrunning team.
Deckers must avoid restricting their options to solely hacking within the Matrix. Although paydata is usually only available via decking, when it comes to scouting or reaching otherwise inaccessible areas, sometimes drones can do the job more easily.
In meatspace, deckers also have an unique ability called Mark Target, which makes it easier for their team to hit the enemy. Curiously, this ability can be used even if the decker does not have a cyberdeck equipped.