Rhythm is the beat or pulse in music, dance, song and poetry. It may be regular and simple like a waltz (1, 2, 3 - 1, 2, 3) or complex such as a piece in 10/8 time.
Within the basic rhythm, there may be a pulse which is highlighted by emphasis of certain notes (like the first) or by the lengths of notes e.g slow, quick-quick, slow. Silences too play their part in rhythm and there may be complex interplay between different beats.
A Waltz may go 1, 2, 3 .... with the emphasis falling on the first beat while ...
A Mazurka may go 1, 2, 3 ... or 1, 2-and, 3 with the emphasis falling differently
A Jig on the other hand is two groups of three 1,2,3 - 2,2,3 and therefore has a double rhythm of the two groups and the triplet of notes inside each group.
Beats may be varied in strength. For example in some pieces in 4/4 time, the first beat is emphasised most but the third is emphasised but less so - 1,2,3,4 .
By varying the duration of the notes, many different rhythms can be derived form the basic rhythm or timing. The common time or 4/4 time is well known as a Reel but if the third beat is split in two, it becomes a Cha Cha (with suitable Tempo of course) - 1,2, 3 and 4 or cha cha cha-cha-cha.