Bylaws define how a nonprofit will operate and the duties of the board directors.  They outline how your organization will be governed. Your Bylaws ensure you are acting legally and in a way that is consistent with your organization's mission.  Never ignore the Bylaws.

Bylaws define how the organization will operate, such as: 

• number of members on the Board of Directors
• length of the terms for all officer positions
• how meetings are conducted 
• responsibilities of the board committees

After your organization creates Bylaws, make sure to keep them up-to-date. You must change your Bylaws to keep pace with organizational changes that happened over time. 

For example, you might need to change your Bylaws if you are offering new services to a new population, if your organization has recently adopted a new mission or if there are new state or federal laws that govern nonprofits.

Sources
A Legal Handbook for Nonprofit Corporation Volunteers: http://www.iciclesoftware.com/VLH7/
“Starting a Nonprofit,” Foundation Center: foundationcenter.org

Resource
You can find an example of Bylaws at: http://www.floridaliteracy.org/toolkitfiles/SampleBylaws.pdf