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 Premiere or Final Cut Pro.
You can trim, edit, split and combine videos right in QuickTime. Here's how...
⌘Y or Edit / Split Clip - This will cut the clip in two. You can then click on one of the sections hit the delete key to delete it.
Drag an video file from your finder onto a video that's opened in QuickTime. It's automatically added at the end.
Save Edited Video
Once you split or combine videos it will create a new "untitled" video. Simply close this new video and it will prompt you to save as a new file.