Here's a powerful and helpful feature of QuickTime that not many people know about. It will save you a lot of time be letting you do the basic edits to videos that you need without using the more time-consuming and advanced editing systems like Adobe Premiere or Final Cut Pro.
You can trim, edit, split and combine videos right in QuickTime. Here's how...
Splitting a clip
⌘Y or navigate to Edit / Split Clip - This will cut the clip in two. You can then click on one of the sections hit the delete key to erase it.
Drag a video file from Finder onto a video that's opened in QuickTime. It will be automatically added to the end of the video.
Saving edited video
Once you split or combine videos it will create a new "untitled" video. Simply close this new video or press ⌘S and it will prompt you to save as a new file.