covalent bonds

Please explain difference between pi and sigma bonds to me. I've been reading chemistry all day long and my brain just can't absorb anything more from books so maybe someone can explain it simpler.

All single bonds are sigma bonds.  Sigma bonds can rotate.  Therefore, single bonds can rotate.  (there are other factors that restrict bond rotation, but for now we'll leave it at that)

A double bond is made of a sigma bond and a pi bond.  The pi bond imparts an inability for that bond to rotate.  This is why double bonds do not rotate.

Why is it that pi bonds impart an inability to rotate?  Look in your book for a picture of two pi orbitals overlapping each other.  It should be obvious why you cannot rotate this bond.  To force the bond to rotate would mean breaking the pi bond.  Since that isn't energetically favorable, double bonds - having one pi bond - are said to be unable to rotate.

Same goes for triple bonds.  Triple bonds are made of one single bond and two pi bonds.  The same logic follows.