It's the same difference as between "this" and "that", except in the plural. "These" is used are used for something physically or emotionally closer to the speaker, and "those" otherwise. Like "this" and "that", the words can be used either as pronouns or determiners.
For example, if you have a handful of foreign coins in your hand, some of which you don't recognise, you might point to them and ask, "What are these?". But if you see birds flying which you don't recognise, you would ask, "What are those birds?"