If you're like me, you've scratched your head wondering how to grab WordPress posts from more than one category or tag. More specifically, not just posts from several categories but from overlapping categories--"so if category x and y are chosen, it will not show posts from category x and category y, but only those that belong to both categories." Despite the fact that this seems like such a simple and obvious feature, it's not part of WordPress's core functionality. I tried hacking functions. I tried doing surgery on WordPress's bowels. Then I found the Intersect plugin, and punched myself in the face for wasting so much time. I hope this post helps someone else find the plugin faster than I did (it was buried in the WP plugin databased under the "misc" category and written in French).