Quantification is an expression of the quantity (roughly: how many?) of entities that have a property. Quantifiers — terms that express quantification — exist in natural languages (“few”, “some”, “all”) and in formal languages like CycL.
For example, the difference in meaning between “{ Few / Some / Many / All } ontologists are rich” illustrates the different sorts of quantification expressed by “few”, “some”, “many”, and “all”.
Existential and universal quantification are paradigmatic varieties of quantification.
In formal languages, quantification typically involves a quantifier expression and one or more variables (which are bound and “quantified over” by the quantifier expression).