What is the difference between LLC and an S-Corp? Can an LLC be an S-Corp or can an S-Corp be an LLC? And how can an LLC be taxed as an S-Corp?

I am sorry if this is 3 questions in one, but it is something that I have been confused about for a long time and have not found a clarifying answer.  I hope this question (series of questions), will help other people understand as well.