Hip Hop dance refers to dance styles primarily danced to hip hop music or that have evolved as a part of the hip hop culture. This includes a wide range of styles such as breaking, popping, locking, and krumping. Breaking, locking, and popping were developed in the 1970s by Black and Latino Americans. Krumping followed in the 1990s and was developed by Black Americans in Compton, CA.
What separates hip hop dance from other forms of dance is that it is often improvisational (freestyle) in nature and hip hop dancers frequently engage in battles—formal or informal one on one dance competitions. Freestyle sessions and battles are usually performed in a cipher, a circular dance space which forms naturally once the dancing begins. It was DJ Afrika Bambaataa that outlined the five pillars of hip hop culture including breaking as one of them (along with rapping, DJing, beatboxing, and graffiti).
The dance industry responded to hip hop dance by creating a more commercial version of it. This "studio hip hop", sometimes called new style is the kind of hip hop dancing seen in most rap and R&B music videos. Technically speaking, hip hop dance (new style hip hop that is) is characterized as hard hitting. The feet are grounded, the chest is down, and the body is kept loose so that a dancer can easily alternate between hitting the beat or riding through the beat. This is in contrast to ballet or ballroom dancing where the chest is upright and the body is stiff. In addition, new style hip hop is very rhythmic and there's a lot of emphasis placed on musicality—how sensitive your movements are to the music.