A striking difference between Argentine tango and ballroom tango is in the shape and feel of the embrace. Ballroom technique dictates that partners arch their upper bodies away from each other, while maintaining contact at the hip, in an offset frame.
In Argentine tango, it is nearly the opposite: the dancers' chests are closer to each other than are their hips, and often there is contact at about the level of the chest (the contact point differing, depending on the height of the leader and the closeness of the embrace). In close embrace, the leader and the follower's chests are in contact and they are dancing with their heads touching or very near each other. In open embrace, there can be as much space as desired between the partners, but there should always be complete contact along the embracing arms to give optimum communication. Since Argentine tango is almost entirely improvisational, there needs to be clear communication between partners. Even when dancing in a very open embrace, Argentine tango dancers do not hold their upper bodies arched away from each other; each partner is over their own axis. Whether open or closed, a tango embrace is not rigid, but relaxed, like a hug.
Source: Wikipedia - Argentine Tango