Adaptive bitrate streaming: Client measures bandwidth. Requests appropriate quality. Switches mid-stream if bandwidth changes. Protocols: HLS (Apple), DASH (standard).
CDN distribution: Videos cached at edge locations worldwide. User in Tokyo streams from Tokyo CDN, not US origin. Popular videos are "hot" in cache. Long-tail videos fetched from origin.
Video chunks: Videos split into - second segments. Each segment available in multiple qualities. Client requests next segment at appropriate quality.