What the FCC says -- Title 47, Part 97 Amateur Radio Service Subpart 97.3 Definitions:
(4) Amateur service. A radiocommunication service for the purpose of self-training, intercommunication and technical investigations carried out by amateurs, that is, duly authorized persons interested in radio technique solely with a personal aim and without pecuniary interest.
"You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? And radio operates exactly the same way: You send signals here, they receive them there. The only difference is that there is no cat. " - Unknown.
"The definition of a "good game" is any activity which gives you a feeling of 'if I just try one more time I can do better.' Ham radio is a very good game." - Unknown.
Operating
US Amateur Radio Band Plan Chart, and detailed Band Plan. Note that some features of the band plan are FCC rules where-as the band plans in other countries are often an agreement or suggestion while the regulations are fairly simple in regard to power level, emission type, and bandwidth.
Various regions around the US also have their own modifications to the band plan on the VHF and UHF bands for example, make sure you're aware of any localized adjustments to simplex channels, repeater pairs, etc especially in or near densely populated areas.
The Considerate Operators Frequency Guide (US)
IARU Codes of Conduct
Ethics and Operating Procedures for the Radio Amateur
Don't Be a Curmudgeon
Capture the Magic of Six Meters - eBook - K5ND
Introduction to the terms of contesting
Guide to VHF Contesting - K5ND
FT8 Operating Guide - ZL2IFB
Signal Identification Guide
DLARC - The Digital Library of Amateur Radio and Communications is a library of materials and collections related to amateur radio and early communications.
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"Some ops like square knobs"