A video highlight is usually a short edit of the video that includes the most interesting or important parts. For example, after a sports event some channels may show a quick "highlight reel" to viewers who want to see what happened, but only care about the significant moments.
A video clip, on the other hand, is a single, usually unbroken or unedited section of the video. YouTube is full of various clips of perhaps tens of thousands of popular movies.
Naturally a clip can be of some highlight from the video, such as this iconic scene from the movie Titanic.
Lastly a video/movie trailer is a promotional edit of the movie which shows the actors in the movie, as well as certain enticing moments. A good trailer should make the viewer want to see the movie, without revealing any important plot elements, such as this one from the latest "Mission Impossible" movie. A shorter version of a trailer is called a teaser (although the line between the two is pretty fuzzy).
[Edit] In response to OP's comment: If you're looking for the name of some event that's part of the actual video but not related to the main action, you might use the term sideshow. This originally was used in things like the circus, where you'd have one thing going on in the "main" part of the show, while other things were going on to the sides, or outside the show.
However sideshow can be used for anything that distracts from the main action, such as this usher dancing at a basketball game, or this dog stealing the ball during a soccer (football) match.
If the sideshow (or any secondary act) is more interesting than the main event, we can say that person (or scene) "steals the show". This often happens in movies where the main actors may be good, but one of the character actors turns in an amazing and memorable performance that dominates the movie.