What is the difference between a USP and a slogan?

What's the difference between a Unique Selling Proposition (USP) and a slogan?

I'm thinking the USP is more descriptive. I have a slogan that I think is really catchy and descriptive in conjunction with the business concept, but standing by itself it really isn't clear what we do. I'm not sure if that makes it bad or not. Are these actually two different things?

I have a logo I came up with

I'm guessing you mean slogan. So I'll base this response on that.

Your slogan is your primary pay-off line. Often catchy, often has something to do with your domain and some of your USPs. This becomes part of your brand. e.g. FedEx - The world on time; Microsoft - what do you want to do today?

Your USPs are what makes you different. These are what will let you carve a special place for yourself in the market.

Check out this video for a great talk on how your USPs are core to getting out of the pack: http://blog.businessofsoftware.org/2010/03/video-of-geoffrey-moore-at-business-of-software-2009.html In a recent talk in South Africa, Dr Demartini said "Write down 200 benefits your business delivers to an ever growing number of people".

Among those benefits will be your USPs as well as regular value propositions. This cloud of benefits should be able to tell you if your slogan is relevant. And you can always change it after a while.