This is a reference request question. I would like to know more on the structure of low dimensional nilpotent lie algebras. I heard that up to dimension 6 there are only finitely many isomorphism classes, and every such algebra admits a gradation with only positive degrees (see <http://en.wikipedia.org/wiki/Graded_Lie_algebra>). 
Do you know of any source where I can find the corresponding proofs?