Long-term gamers will recall that in Opposing Force you played Adrian Shephard, one of the special forces sent into the Black Mesa facility to kill the witnesses to the resonance cascade event that permitted the invasion of Earth by the evil Combine in the first place. Shephard didn't take part in the morally murky shenanigans, instead being enlisted by the enigmatic G-Man to help out Gordon Freeman in his mission from behind the scenes. At the end of the game G-Man temporally suspended Shephard like he did Freeman.
In Prospekt, Shephard is awoken after a decade or so in suspension. Earth is now under Combine occupation and the events of Half-Life 2 are underway. Gordon Freeman is mounting his assault on the Nova Prospekt prison facility and Shephard is once again deployed to help him from behind the scenes, diverting Combine reinforcements and so forth. The game pits Shephard against traditional Half-Life 2 enemies (and some from the episodes). The game features updated visual effects because, well, it's not 2004 any more.
Sadly, overeager fans shouldn't read too much into the release of this episode and what it might mean for the long-MIA Half-Life 3. The game is the creation of Half-Life superfan Richard Seabrook. It so impressed Valve that they've agreed to release it officially, and you can get it on Steam here.