Here, let me help you out.
Copy and paste this into your OP if you want. It's an interesting enough topic.
Slate article explaining the decision: http://slatest.slate.com/posts/2011/06/20/wal_mart_sex_discrimination_supreme_court_rules_class_action_can.html
In short, here's what happened.
The plaintiffs represent a group of women who claim that they were discriminated against by Wal-Mart. The substance of their claim is that Wal-Mart's corporate culture encourages sexism; furthermore allowing district managers to make hiring decisions allows that sexist culture to manifest locally without it being directly caused by discrimination on a high national level. They seeked to submit a class-action lawsuit on behalf of all women who worked for Wal-Mart.
SCOTUS told the plaintiffs that they could not proceed with a class-action lawsuit, because they could not show that there was a single common illegal practice that united them as a class. In other words, if one woman had suffered sexual harassment in one state, while another woman had suffered reduced pay due to sexist discrimination in another, then while they were both hurt by sexism, the "glue" that united them as a class was not common enough.
I'm not sure how I feel about this. I am highly sympathetic to the plaintiffs, however, I am also sympathetic to the notion that to proceed as a class demands that the plaintiffs show a substantive common thread - not merely that they were harmed in a vaguely similar manner by a toxic corporate culture.