Hardstyle is a genre of EDM characterized by heavy bass and high BPM counts, typically in the range of 150. It lacks the fluidity of trance, which is the main characteristic that distinguishes it from hard trance. It sounds much like hardcore albeit at (relatively speaking) slower BPM counts.
Hardstyle is currently the music of choice in many European countries, especially Holland. Parties/raves that play hardstyle music include Qlimax, Q-Base (Q-Dance), and Sensation Black (ID&T).
Hardstyle DJs include Luna, Dana, Issac, Walt, Zany, Pavo, etc.
Example: Southstylers - E-Town
A: What's the name of the "I'll be your bitch" track that I just heard?
B: It's DJ Isaac - Backstage. Absolutely rockin hardstyle tune.