Also see "Roof Drainage".
1. General: The Wikipedia article here provides an accurate overview.
3. Maintenance: "General Inspection And Maintenance Of Gutters And Downspouts". Available on the website of the US General Services Administration.
5. Technical: "Gutters and Downspouts" can be seen at the website of the Copper Development Association, and includes an explanation of the process of determining the proper dimensions of a hypothetical gutter. Be aware that the data in the Expansion Joint section is specific to copper gutters.
6. Manufacturers: Gutter manufacturers can be found here.