Many call this game just a Doom clone, but of course it's not as simple as that. Like most games from the early days of the (partial) 3D first-person shooter genre, Dark Forces builds on what was established by its predecessors. It takes Doom's formula, uses it as the foundations and creates something new and improved (just like Doom did with Wolfenstein 3D). From a gameplay perspective it is quite similar, but a number of key features have been added which were quite new in 1995.
In addition to all the typical stuff you'd expect from a mid-1990s shooter, Dark Forces also allows the player to aim vertically (this can however only be done using the numpad; the mouse is used for horizontal aiming instead). Other features added by the Jedi engine are the ability to jump and crouch and a number of atmospheric effects. Some missions require the use of a headlamp, night vision, gas mask, special boots and other tools. The player can also throw thermal detonators (explosives).
Dark Forces is set in the Star Wars universe. Since it is a licensed production, everything from the visuals and sound effects have a distinct feel. If you enjoyed the original trilogy of Star Wars (episodes IV, V and VI), this game will certainly make feel you nostalgic. The levels are slightly less abstract than Doom, but they are still very well designed and quite complicated, also forcing the player to think by adding various puzzles throughout. Dark Forces has a total of 14 missions and during the course of the game you will see aesthetically different environments.
Besides differences in gameplay and visuals, Dark Forces is also more story-focused than the Doom games. You play as Kyle Katarn, a mercenary recruited by the Rebel Alliance, one that used to work for the Galactic Empire. The plot begins shortly before the events of Star Wars Episode IV: A New Hope, but gradually takes you past the ending of the film and takes place between A New Hope and Star Wars Episode V: The Empire Strikes Back. It's not masterful storytelling, but the simple plot and visually pleasant cutscenes make the game more engaging.
Overall, the game's numerous mechanical improvements and technological advancements make it a natural successor to Doom 2, adding variety to the Doom gameplay formula. Star Wars: Dark Forces was a brilliant game for its time; since then it has influenced other amazing titles (such as Duke Nukem 3D) and remains a classic.
NOTE: The Steam re-release of this game isn't entirely up to scratch. Upon launching the game, you will experience terrible performance issues (especially when there are multiple enemies on screen). It's nothing that a quick Google search can't fix, but it should be in-game from the get-go.