Brewing has certainly come a long way since its infancy thousands of years ago. The ingredients to make beer have ranged from grasses to grains to herbs to fruits, etc. Heck, even up until the 1800's, people didn't even know that yeast was indeed an ingredient!
These days, I think most brewers can agree on the 4 main ingredients: water, barley, hops and yeast. I've laid out some explanations in the links below, and will continue to expand on this page as they are covered in more depth.
Water
Malted Barley (or Extract)
Hops
Yeast
