Enterprise is a subset of B2B cos. While this isn't an exact science and the very concept of 'Enterprise' has been disrupted by companies such as box.net, I would say that typically: Enterprise = B2B cos with annual contract values > $100k, which usually means a different sales model, too.
Enterprise has nothing to do with only 500 companies in the US.
There is no real difference.
Though some might say if you charge thousands and have dedicated salespeople, you're enterprise, even if you sell to a 25 person architect firm. Sales force is definitely though of as enterprise. While basecamp or twilio is more B2B. It's fuzzy, if there even is a difference.