Hack and Slash games have been around for a while; even some classic video games like Gauntles, Altered Beast and Castlevania could be considered part of the genre. The premise of these games is to cut up opponents into tiny kibbles in as many satisfyingly gory ways as possible. As hardware become more complex, so did the Hack-and-Slash. Many recent games pit a super-warrior against a literal army of foes, which can be awe-inspring.
There's nothing really complicated about these types of games, and many of them have redundant gameplay ad nauseum. But I guess that's part of the genre's charm. One way developers have made new games more appealing is to churn out the eye-candy, in terms of graphical visuals and scantily-clad heroines.