The operating agreement is a document, usually drafted upon formation, that basically includes all topics covered by a so-called "founders agreement": restrictions to transfer of ownership, profit splitting, what happens if the company needs money, etc.
The founders' agreement, for an LLC, is like the termsheet of the operating agreement. A shorter, less technical agreement between the founders where they agree on the business terms ($$$ in particular) that will be further developed in the operating agreement.
This question was asked on Quora, too. Here is the heart of the answer that I posted there:
An operating agreement applies only to, and specifies the ownership and management of, an LLC.
A founders' agreement can address whatever issues company founders want to document to establish a fair ownership and working relationship and to reduce the likelihood of misunderstanding or animosity down the road. It usually is thought of in the context of a corporation (addressing, for example, vesting, voting and restrictions on transfer of stock), but it could, conceivably, apply to an LLC, too.