tl;dr: LearnOpenGL is a free, comprehensive tutorial resource for modern (core-profile) OpenGL, covering everything from basic graphics programming concepts to advanced techniques like lighting, model loading, and post-processing, culminating in a small game walkthrough. Unlike many older resources focused on immediate mode, it targets modern OpenGL with step-by-step examples suitable for beginners and useful as a reference for experienced developers. The content is also available as a print book on Amazon and other retailers, or as a free PDF.