As per cyberpunk conventions, the player is an impoverished hacker in a dystopian future, stealing files and crashing systems to pay for the monthly rent.
While Decker is supposedly a hacking simulation, the gameplay is actually more similar to a Roguelike. Once in the matrix, the player travels from node to node with various personified security programs roaming about. Some of these programs guard the entrance to a node, querying the player when an attempt to enter is detected (and signaling alerts if the queries are not responded to properly), while others patrol the system looking for intruders. There are also the nastier types that are actually capable of attacking, such as the Trace&Fry programs that will attack and fry your chips in the physical World, and encryption-type programs like the Tapeworm, that guard (sometimes) important files.
The gameplay, like Roguelikes, is not real-time. Each move (which consists of either running a program, performing an action, or moving from one node to another) is expressed as 1 second only for the sake of time-limit-dependent missions.
When not in the matrix, the player can shop for items (in the form of programs and chips) and spend skill points like any ordinary RPG, boosting the character's various attributes (attack, defense, stealth, programming, chip design, etc). The player can also spend some time creating higher-level programs to use or burning higher-ranked chips.
After the successful completion of numerous missions, the player's reputation will go up gradually, making available more difficult missions and higher-ranked software/hardware. At some point, the player would have to upgrade the character's lifestyle (e.g. raising the monthly rent) to allow for further reputation growth.
The missions themselves can involve anything from altering, deleting, and/or acquiring sensitive material, to deactivating alarms, setting off toxic gas, creating back-doors, crashing systems, and running client-supplied programs.
While the game is reasonably addictive in the beginning, the player will soon realize that all missions consist of doing one of four things: locate a certain node (by scanning) and press the 'perform action' button (whether this action is performed to deactivate an alarm, set off toxic gas, or create a backdoor will cease to be important very quickly), locate a certain file (again, by scanning) and do something to it, or load a client-supplied program, locate a node, and run it.
The rest is merely sitting there, performing these repetitive actions, and watching your stats go up.
It is, however, especially suited for coffee-breaks.
There are two ways to lose the game: by being brain-fried to death from fights with security programs, or by extreme poverty (i.e. not being able to pay the monthly rent). Neither losing conditions are easy to achieve.
While the game has the potential to offer high replayability, it is not well developed. Stealth will soon outrank everything else in terms of importance, and developing anything other than a stealth-based character will be of interest only to those who like to push limits and set self-imposed goals.
While the graphics are by no means top-notch, there is a reasonable amount of detail, especially for a semi-Roguelike.
There is no music, and the sounds played when an action is performed in the matrix are not all that well-made. They are, however, helpful for the player by being able to express what is happening in the game, especially when the repetitive nature of the game sets in and the player begins to recklessly charge around the system without bothering to smoke a room for that stealth bonus. I have personally ignored several queries by accident while speeding through missions (and ended up being fried) when playing with the sound turned off.
1. In the matrix
2. Mission selection
3. Home screen and programming