In any language, you can divide words in three categories:

1. Words of foreign origin
2. Words derived from roots
3. Words roots that are unbreakable

1. Words of foreign origin
These are either international words such as hotel, taxi, restaurant or not so international words imported from a neighbouring language, such as German Phantasie, Bürokratie, Portemonnaie or English rendezvous, faux pas, debutante (all imported from French). The more polyglot you are, the easier these words are to learn in your new language, since you are likely to have met them before. Some of these words are bound to be false friends - they look the same but their meaning is different, such as English entree (main course) from French entrée (first course). More about cognates ...

2. Words derived from roots
In any language, you can break down most words to a fairly limited number of shared roots. The opposite is not true and you can hardly make up new words based on the roots. Even telling what a word means from its roots is usually not so easy. The point is that you can use these roots to help you remember a new word rather than having to learn by heart every word as if it were a random string of sounds. For instance, French for 'good friend' is copain. If you realize that 'co' means 'with' and 'pain' means 'bread', and that in French the copain is the person you share bread with, remembering the word becomes easier. More about words derived from roots ...

3. Words roots that are unbreakable
These are the truly unique words of a language. They are unique in the sense that you cannot derive one of these words from another in the same language. However, you can often find close equivalents in related languages. For example, Russian for brother is brat, close enough to German Bruder and even fratello in Italian. All come from a common Indo-European ancestor. You don't need to have a PhD in linguistics to take advantage of this to learn vocabulary faster - just a keen eye.

Instead of looking at new vocabulary passively like cows watch trains passing by, you can use these concepts to actively find ways of relating the new words to something you are already familiar with. This will reduce the effort required to remember them. This is just one way to make vocabulary acquisition easier and you can definitely combine it with mental hooks for instance.

This classification of vocabulary was first introduced to me in a book for American mathematicians wishing to read Russian mathematical articles.