On step sequencers, musical notes are rounded into steps of equal time-intervals, and users can enter each musical note without exact timing; instead, the timing and duration of each step can be designated in several different ways:

• On the drum machines: select a trigger timing from a row of step-buttons.
• On the bass machines: select a step note (or rest) from a chromatic keypads, then select a step duration (or tie) from a group of length-buttons, sequentially.
• On the several home keyboards:in addition to the realtime sequencer, a pair of step trigger button is provided; using it, notes on the pre-recorded sequence can be triggered in arbitrary timings for the timing dedicated recordings or performances.

Information courtesy of Wikipedia