A great definition by Rob Thomas on Huffington Post:
A civil union has to do with death. It's essentially a document that gives you lower taxes and the right to let your faux spouse collect your insurance when you pass away. A marriage is about life. It's about a commitment. And this argument is about allowing people to have the right to make that commitment, even if it doesn't make sense to you. Anything else falls under the category of "separate but equal" and we know how that works out.