Traditionally, an organization would have two official documents. The "organizing instrument" is usually a Constitution, Charter, Articles of Organization, or Articles of Incorporation. This defines what the organization is and why it exists. It usually has the very bare bones such as the name, purpose, basic organizational structure, fiscal year, policies, and how amendments can be adopted.
The bylaws are a companion document that state the rules by which the organization operates. They are much more detailed.
When I first started researching this several years ago, the set was what I understood was the "right" way to go about it. Since then, I've discovered many PTO's have a composite document, generally just called the bylaws. When we were getting our 501(c)(3), the IRS quite firmly seemed to expect us to have an organizing document besides the bylaws (for us, our constitution), so I'm not sure how others with a combined document have gotten around that. Perhaps the IRS has gotten more lenient.
You might think of the organizing instrument as a document positioned more for outsiders - to legally define who and what you are. An outsider, like state officials or the IRS wouldn't necessarily care about the day-to-day rules (bylaws). Those are intended more for internal use.
In choosing an "organizing instrument", the key question is whether you plan to incorporate (which is a good idea). If so, you want to find out what the Articles of Incorporation need to contain in your state. For groups like PTO's, it's usually pretty simple. In Texas they provide a 1 page template that can be used if you don't have your own.
Once this is filed, people tend to forget about it because it's the bylaws that always get referenced. But this would equate to the organization's birth certificate, so it's not exactly unimportant - just not something you need every day.
[ 05-12-2004, 11:56 PM: Message edited by: JHB ]