What is WebVTT?
The subtitle format built for HTML5 video - what a browser loads into a track element. It's SRT with a header, a full stop instead of a comma in the timestamps, and optional positioning and styling that most people never touch.
What is SRT?
SubRip subtitles: a number, a timestamp, a line of text, a blank line, repeat. That's the entire format. It carries no styling and no positioning, which is precisely why every player, TV and video tool on earth reads it.