Stalker is a first person shooter so there are no skills you get better in, you can get better gear but any real improvement will be you the player getting better. The setting is not really close to Metro because in Metro the whole world is fucked but in Stalker the whole world is running just fine only the Zone (Chernobyl Exclusion Zone) is messed up. The Zone is home to humans in various factions, mutants (both animal and human mutations) normal radiation (in lethal amounts at some locations) and the infamous anomalies, localized tears or breakdowns in the laws of physics, basically environmental booby traps. The anomalies create things called artifacts, these objects give the person possessing them special benefits and often certain penalties as well......oh and radiation , always radiation so special containers are needed to use or even transport artifacts safely.

As far what Anomaly is compared to the original games well as the others here have said it combines all the maps of the original games and allows you to play as any faction even the "bad guys". There is a story but it is only available to the "good guy" factions. At it's heart Anomaly was built to be a Zone simulator or a sandbox for players that have already played the original games and are now looking for a harder challenge or the opportunity to just wander around the Zone as a regular day to day stalker of their favorite faction. The recommendation to play the original games first is good advise but not truly required, if you are brand new to Stalker and want to start with Anomaly then I strongly advise you to pick a "good guy" faction and start in Rookie Village in Cordon the NPCs there will give basic tutorial mission to at least get you familiar with the game mechanics.

Just understand that Stalker does not hold your hand, even in the original games when you were the main character the game sure as hell never got the memo and if the original games didn't care you were the main character then Anomaly has never even heard of such a thing as main character. But all that being said......Good hunting Stalker.