Sounds may or may not have pitch. Many percussion instruments do not have pitch yet function in the rhythm section in popular forms of music: they create and maintain the beat. Tango, as far as I know, is unique in not having a dedicated rhythm section.
Musicians call sounds with pitch, s, and silences, rests. A note has both pitch and duration; a rest has only duration. Note and rest duration is called time value or note value. When notes, and possibly rests, of different time value are combined there is rhythm. (Usually different time values, but not necessarily).
A rhythmic pattern is a small grouping of note (and rest) durations. Rhythmic patterns themselves are often repeated. Repetition in music creates a sense of structural continuity and cohesiveness. Rhythm and rhythmic patterns directly contribute to the effect of movement in music. The repetition of a rhythmic pattern creates a sense of forward motion by the appearance of successive sounds in time. The interplay of different note and rest lengths is perceived as motion by the listener.
In tango (and most popular music) there are two broad rhythmic elements, or layers operating simultaneously: melodic rhythm and accompaniment rhythm. Melodic rhythm identifies the rhythms used in the melody. They may be any combination of time values, sometimes using the and its variations. Accompaniment rhythm may be called the beat, when it is marked (played) in regularly spaced time intervals. When 4 beats per bar are marked tango musicians call the rhythm “marcado in 4″. And when 2 beats are marked – 1 and 3 – it is called “marcado in 2″. Other accompaniment rhythmic patterns are s, the habanera, and patterns derived from it, the syncopa, and the 3-3-2.
Usually the two rhythmic layers are differentiated: melodic rhythm is independent of the accompaniment’s marking the beat or its syncopation. Sometimes both layers are combined into a single, unifying rhythmic pattern.
Beat is the underlying and regularly spaced pulse of the music, measured in beats per minute. There are a fixed number of beats in a bar, indicated by the time signature. Tango (2/4, 4/8, 4/4) has 2 or 4 beats per bar, vals (3/4) has 3 and milonga (2/4) has 2.
(There may be a sense of 4 beats even though the time signature is 2/4. Tango very often subdivides the 2/4 beat, doubling the count from 2 to 4, effectively using a 4/8 time signature. Some tango music is explicitly written in 4/8, most are in 2/4. See Tango Time Signatures and the Beat).
A note is a sound or tone having two aspects:
1) The primary, auditory one, is pitch. Each note has a unique pitch, with a sound wave frequency measurable in hertz. In many parts of the world instruments are tuned to A at 440Hz.
2) The secondary, temporal one, is duration, called "time value" or "note value". When written or played each note has a specific duration, how long it lasts relative to the beat.
When pitch and duration are combined we get melodic shape and rhythm. Pitch creates melody and gives it direction, the melodic shape; duration provides the melodic rhythm.
Milonga came before tango and its characteristic rhythm was, and is, the Cuban habanera. Early tango's rhythm was based on the habanera also. Soon other syncopated patterns dominated. The 1st variation, the "syncopa" is the most commonly heard syncopated rhythm in tango music. All three versions are used in milonga.
Habanera Variations describes the syncopation and relationship between these patterns.