"Hard core" can mean several different things depending on the context. Typically, it refers to either gaming or pornography. A "hard core" gamer is someone who spends a lot of time getting very good at a video game. It's also used as a perjorative (that is, an insult) to mean "snobby, elitist, angry, and antisocial". As for porn, there's a distinction between softcore and hardcore. Softcore is generally "nice, tasteful" pornography, while hardcore porn would be very explicit, detailed, and (depending on the situation) grotesque. Lesser uses of the term could be found in party scenes (people who party all night long would be "hard core"), music genres (hard core gangster rap), or even some blue collar professions. Essentially, it can either mean "extremely dedicated", "extremely violent", or "extremely detailed" depending on the context (and it's sometimes a combination of all three).

Hard core can mean: (1) regarding pornography: explicit; very detailed; showing everything. "You can find hard core pornography all over the internet. (2) extremely dedicated; fanatical: "He is a hard core terrorist."