A founder is a person who comes up with an idea (hopefully a profitable idea) and then transforms it into a new business or startup. Founders can set up a business on their own, or they can do it with others (what we call cofounders). For example, Larry Page is a founder of Google.

If a founder sets up a company with other people, they are both a founder and a co-founder. Let's use Google to illustrate. So, Larry Page is not only Google's founder, but also a co-founder with Sergey Brin. Cofounder is a term that exists to give equal credit to multiple people who start a business together.