What 3/4 and 6/8 have in common is that they both allow for 6 eighth notes in a bar.Where they differ is that in 3/4,the eighth notes would be counted "One and Two and Three and" wheras in 6/8 They would be "One and a Two and a". In other words,in 3/4 you have 3 pairs of 2,and in 6/8 you have 2 groups of 3,so it has more of a triplet feel.