Determiners are a group of words which include articles (a, the), demonstratives (this, those), possessive adjectives (my, their), numbers (one, two), and other words (any, both) that come before a noun, follow another determiner, or begin a noun phrase.
Determiners include quantifiers, which are words or phrases used before a noun to show its quantity. Examples of quantifiers are: all, many, much, most, one, some, a few, and a lot of, etc.
Determiners and quantifiers are useful in letting us know more about the noun that they refer. For example, this thing identifies something that is close at hand; her wig means the wig belongs to her; another donkey tells of one more donkey; and a few marbles refers to a small number of marble.