If you achieve a share ratio of 1:1, that would mean you've uploaded as much as you've downloaded. This is typically a minimun acceptable result. The higher the number, the more you have contributed. While share ratings are just displayed for your convenience, courtesy to others should cause you to keep this ratio as high as possible in the event you need to prove you're not a leecher.
4 guys and 2 girls is a bad bd-ratio, and is a .5
6 girls and 2 guys is a great bd ratio, at 3.0.
"Man, this 2.0 ratio is awsome, gurranteed 3 ways for every dude!)