In the study of English grammar, the word demonstrative is a technical term. Demonstrative words are words that show (demonstrate) what you are talking about: they serve to identify what is being talked about by indicating its "particular spatial or temporal relationship with the speaker or writer" (The Oxford Dictionary of English Grammar, Bas Aarts, Sylvia Chalker, & Edmund Weiner (eds.), 2nd ed. OUP, 2014). The commonest demonstratives are:
- this and its plural form these
- that and its plural form those (but to complicate matters, that can also be used as a conjunction)
They can be used as either adjectives or pronouns: we speak of demonstrative pronouns or demonstrative adjectives.
- a pronoun stands for a noun. So a demonstrative pronoun is a unit on its own ("Do you like this?");
- an adjective qualifies (describes) a noun. So a demonstrative adjective precedes a noun or a pronoun ("I like this one", "I like these bananas").
This distinction does not really matter, most of the time, unless you are an expert - or working with two different languages. Most of the time, "demonstrative" may be used as a noun: a student might be told to "Underline the demonstratives in this sentence", and it would not matter whether the words were being used as adjectives or as pronouns.
Demonstrative words are a closed set: no new demonstratives are likely to be invented. The only one missing from the list above is the archaic yonder.