Swing / straight
There are two main categories of rhythm depending on how each beat is subdivided. The most common division is called straight where each beat is divided equally into one, two or four parts. Notice how the following two examples have the same notes and chords played with different feels.
Straight
Swing involves an unequal division but usually retains an equal number of notes. Swing involves breaking each beat into triplets and then omitting the middle one so that there are eight notes per bar (in 4/4) but not eight even notes. They become grouped into twos.
This is often notated at the top of sheet music as:
An easy way to remember the difference between swing and straight is to visualise these movements:
Walking suits straight as it is even.
Skipping suits swing as the grouping into two's becomes apparent.