Hello Fabio, I wonder this answer shows you correctly because my English is still poor. Generally I use 又 for 2 meanings as Sherry said . One is "Again" for example 又会いましょう(mata aimashou) "see you again" and the other is "Additionally". I usually use 再 for 1 meaning "Again" for example, 再び(futatabi). Please ask me again, if you like.

Thank you very much! Your answer is really interesting, let's see if someone explains it to us.

I found it's interesting that you are asking this since those Kanji are used a lot in Chinese and native speaker of Japanese often ask the exact same question when they try to study Chinese. Anyway, although both characters mean "Again, or something repeated" in Chinese. 再 usually refers to thing that hasn't happened, something in future. I will come again. 又 is usually used to talk about things that already happened past" you came to see me again. " However, such difference probably doesn't exist in Japanese and since if there is a difference between those two in Japanese, people from Japan wouldn't have asked me about it. Again, I could be wrong on that. Now, I am curious about the answer myself too.