Cadences are musical formulas that function like punctuation marks in a sentence. These formulas are conventions that have developed over time to signal the end of a musical unit such as a phrase. Cadences involve the interaction of melody, harmony, rhythm, and meter, and usually mark the end of a phrase. Some cadences sound conclusive, complete, and resolved (like a period), while others sound incomplete (like a comma or colon). The most complete type of cadence is the full cadence, which moves from V to I. In a half cadence the progression is stopped on V, which makes it feel unfinished.
Sometimes composers will even start a cadential formula, but instead of arriving at the cadence as expected, they will interrupt or change the formula with deceptive motion.