What are the biggest benefits of using INDEXES in mysql?

Question

I know I need to have a primary key set, and to set anything that should be unique as a unique key, but what is an INDEX and how do I use them?

What are the benefits

Answer 1

Think of it this way: what are the biggest benefits of having an index in a book? It's much the same thing. You have a slightly larger book, yet you're able to quickly look things up. When you create an index on a column, you're saying you want to be able to reference it in a where clause to look it up quickly.