Difference, if any, in S-Corp & LLC?

I am helping a friend put his old business on the web. Another friend suggested that we make the company an S-Corp or an LLC...I only have a vague knowledge of this stuff...what exactly does it mean, and what are the difference between them? I know it allows you to have a tax ID number with the government and so on, but can anyone explain S-Corps and LLCs in more detail? Serious answers only please!!! Thanks!