A founders agreement is a contract between the founders of a company that establishes the terms of their relationship. This can include everything from how the company will be governed to how decisions will be made and what happens if a founder leaves the company.
An operating agreement is a similar contract, but it is between the shareholders of a company and establishes the rules for how the company will be run on a day-to-day basis. In many cases, a founders agreement will be rolled into an operating agreement as the company grows and more shareholders come on board. However, it is important to understand that these two documents are not interchangeable and serve different purposes.