[agfali]

I would be very greatful if someone could help me, I just don't get this buffer thing.. 

I have a 10% (30g/270g H2O)-solution of glycolipid, that has a COOH-function on its fatty acid chain. Molecular weight of the glycolipid is 623 g/mol.

I add 50 mL of 1.0M KOH until the glycolipid is dissolved totally, then I add 40 mL of 1.0M acetic acid - then pH is 5.5.

What things form a buffer in the solution? Acetic acid and potassium hydroxide? And what is a role of water? What about the glycolipid?

This feels so difficult for me.. Thank you already in advance 

[stamba]

Ok. So buffer is practicaly everything in your solution. 
KOH reacts with glycolipid and gives R-COOK salt which with glycolipid gives a buffer.
Next, KOH reacts with acetic acid giving CH3COOK. This salt (CH3COOK) gives buffer with acetic acid. And water is a media (solvent).

Why there are two buffers in there?
Simple, since acetic acid and glycolipid are both weak acids, and theory sais that
buffers are mixtures of weak acids and their salts.
So, here you have two buffer solutions, mixed into one, giving the pH = 5.5.
This VERY, VERY simple explanation, it is far more complexed than that.