Zouk, or Zouk-Lambada Dance is a Brazilian partner dance style.
It is danced to 4:4 time music, with the movement emphasis on the extended 1 count, and transitions and styling on the 3 & 4 counts.
It's one of the more complex dance styles, in that it is highly dynamic, operating in all 3 dimensions with a lot of counterbalance and physicality.
The range of music used for zouk is enormous, everything from classical to hip-hop to pop music, and allows are an intense range of feel and styling.
Zouk has gained popularity worldwide, and in practiced and performed in both "freestyle" and choreographed forms.
Freestyle. None of the movements are planned in advance, and event the music may not be known before it begins. The dancers may never have even heard it before. The leader in the dance must be very decisive and clear, and the follower very responsive and aware, for the dance to work.
Choreography. The music, and all of the movements are planned in advance by both partners. Each knows exactly what will happen next.