Zouk – also Brazilian zouk, Rio zouk, Zouk-Lambada – is a sensual modern couples dance originating from Brazil, with its roots in lambada and Caribbean zouk music. Movement-wise it has influences from lambada, bolero and Samba de Gafiera, and I'd argue also from tango, salsa, contemporary dance, ballet, and even capoeira. Music-wise, zouk is extremely diverse, with remixes spanning everything from pop to hiphop, from R&B to dubstep.
Zouk is rather challenging to learn (especially without prior dance experience, like I was), and especially for leaders who (ideally) lead every single step, tilt and movement of the head. However, once you get over the hump, manage to relax and get out of your head a bit, and establish good connection with your partner, it can be truly magical to dance.