Demonstratives are the following adjectives and pronouns:
These go in front of nouns.
Me gusta este libro. → I like this book.
¿Tienes estas monedas? → Do you have those coins?
These replace nouns. Notice that pronouns have and accent mark, but adjectives don’t. An easy way to remember is that these demonstratives always need either a noun or an accent mark, but never both.
Me gusta éste. → I like this.
¿Tienes éstas? → Do you have those?
When you don´t know what something is you would ask
¿Qué es esto/son estos? → What is this/are these?
¿Qué es eso/son esos? → What is that/are those?
¿Que es aquello/son aquellos? → What is that/are those?
Notice that the demonstrative pronouns don’t have an accent mark when they are neuter.
Ese vs aquel
Both of these translate to the same word in English, that. The difference is simply distance.
ese → that (right there)
aquel → that (far away)
Latter and Former
When comparing to things, éste means the latter (the lastest mentioned) and aquél means the former (first mentioned).
Jorge es mayor que Fernando; éste (Fernando) tiene ocho años, aquél (Jorge) tiene diez.→ Jorge is older than Fernando, this one (Fernando) is eight and that one (Jorge) is ten.
Definite articles as demonstratives
Definite articles (el, la, los, las) that are followed by the word de (the one of, that of, those of) or que (the one/ones that) act as a demonstrative.
el/la de Juan → that of (the one of) Juan
los/las de Josefa → those of (the ones of ) Josefa
el/la que está aquí → that one that is here
los/las que están allí → those ones that are there