It is tango danced to Waltz music (more precisely, Viennese Waltz played with tango instrumentation).
Either in open or close embrace.
Danced in a smooth, continuous and flowing movement - no stoccato.
Played and danced in 3/4 beat rather than the standard 4/4, with the accent on the first beat.
Lots of circularity and rotation: incorporates the calesita (a carousel, where the leader takes the follower on axis and steps through a circle or arc around her) and molinete (windmill, in which followers dance small circles around the leader).
The vals is often called vals cruzado because in order to turn around each other, the lead and follower have to move into cross-step.