What is the difference between 単語 and 語彙？
I know that they can both be used as "vocabulary", but i don't understand in what kind of situation to use each of them to mean that.
Examples would be much appreciated!

Question

Answer

It's very simple. Basically both words are different. "単語" means "word", and "語彙" means "vocabulary". 
For example, you could use those 2 words like the following sentences.

この単語(たんご)の意味(いみ)は何(なに or なん)ですか？ ＝What does this word mean?

英語(えいご)を勉強(べんきょう)して５年(ごねん)になりますが、まだ語彙力(ごいりょく)に(or が)乏(とぼ)しいです。 
＝I have been studying English for 5 years now, but I still have a weak vocabulary.