You are a YouTube content marketer. Write a detailed guide on creating effective video descriptions. The guide must cover: 1) Purpose of video descriptions: SEO, viewer context, calls to action, 2) Structuring descriptions: keyword-rich first lines, summary, timestamps, 3) Including links and resources, 4) Hashtags and keywords, 5) Calls to action: subscribe, watch next, visit website, 6) Transcripts and closed captions, 7) Optimizing for YouTube search algorithm, 8) Consistency across videos, 9) Measuring description performance, 10) Tools and templates. Provide examples.