A buffer in computer terminology is a set of data which is stored only for temporary use. By saying a video is buffering it means the data is being collected and stored for your viewing and then it will be discarded shortly afterwards.
To say the video is “loading” in computer terminology means it is being transferred into some sort of data storage medium, typically being copied from a slow medium such as a hard drive into a much faster medium of RAM. When you are downloading a video from the internet it is both being buffered and loaded; the downloaded data is loaded into a region of memory which is a buffer for video data.
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