I'm not even a hardcore fan here, so I imagine people with deeper knowledge of the series could perhaps better elaborate on the identity of the series over time better than I could.
Universe-wise, they're homages to all kinds of classic horror/action movies, albeit 'legitimised' by being grounded in science fiction. Capcom has managed to eke out many recreations of horror sub-genres by being creative with numerous settings and story lines. But they all typically involve-
- Sexy, charismatic young heroes who have had some kind of military expertise, or at the very least know how to handle a gun, working in pairs
- Insidious and shadowy bioweapons corporations creating horrific, destructive monsters
- A motivational theme of heroism, of pushing forward against adversity despite the odds, with dire consequences for those who 'give up' or display poor moral fibre
- A mystery which unfolds from some sort of initial ornate/gothic setting and culminates with a showdown in a lab or similarly high-tech lair
- Lots of guns
Gameplay-wise, they're about sustaining a double-edged sword illusion: That you can be killed easily, but you're also capable of killing just as effectively. Player movements/animations are also often slowed/delayed/exaggerated to heighten tension, and enemy attacks likewise, but the games are always smartly designed around it. Limited items also make the player feel cautious, and wary of screwing up. Series creator Shinji Mikami basically sums up the appeal of Survival Horror as being that moment where your back is to the wall with low health, and you pull off that lucky shot, killing the enemy. It's an exhilarating loop of awkward failure and sweet success, which with practice, soon becomes stacked in the player's favour.