There are basically three types of Tango dances. Argentine Tango, International Tango and American Tango. American style is much more of a social dance with open partnership positions as well as closed and is completely leadable (the lady doesn't have to know the choreography).
American Style Tango is a smooth dance that travels around the floor counter clockwise. Tango has a modified smooth frame maintaining level shoulders at all times. The biggest difference is that you dance much closer together requiring the man's right hand to be down in the middle of the ladies back and her left hand at the side of the man's shoulder.
The basic rhythm is Slow, Slow, Quick, Quick, Slow. However, there is more than one type of Slow. In general a Slow step is the changing of weight from one foot to another using two beats of music. A Foxtrot Slow is a constant even movement for two beats. A Tango Slow is moving on the first beat and holding the second beat. Tango has a very staccato movement, not constant and even. Tango is the dance of stops. It is often referred to as a cat stalking it's prey.
I like to call it the Grocho Marx dance because you stay bent in the knees for the whole dance. Keeping bent knees prevents any rise or fall or sway which maintains the Tango character.
Musically it is played in 4/4 time at about 30 to 33 MPM (measures per minute). A classic American Tango is "Hernando's Hideaway".
The following are two basic Tango patterns: