You probably already know how to upload videos to YouTube from your Mac, but what about downloading videos anyway? Read this article for a step-by-step explanation of a simple method.
Steps
Step 1. Launch Google Chrome
This method requires Google's browser, which requires Mac OS X 10.6 or later. The method basically boils down to copying the video from Chrome's cache. The method does not work with Safari, Firefox, Internet Explorer or other browsers.
Step 2. Select the YouTube video
Go to the website and find the video you want to download. Open the video, pause the video, and wait for the video to load completely.
Step 3. Open the 'Library' folder on your hard drive
Locate the "Cache" folder and open it. Then click on Google → Chrome → Default → Cache. This folder contains all the recent information that is cached in Google Chrome.
Step 4. Search for the video file
This can be just as difficult. Because this concerns raw data, you will see that the names are difficult to recognize. Scroll up from the bottom of the list as you search for a file that is approximately the right size. Depending on the length and quality, an average HD video will be about 20-40 MB. When you've found the right file, right-click it (or Control-click if you have a single-button mouse), copy the file, and then paste it to your desktop or another location of your choice.
Check the size of the video in YouTube under the gear icon in the lower right corner to have an indication in advance. For example, a video in HD720p will be quite large. If you already know the size of the video before you search the cache folder, you will find the video much sooner. You can probably ignore anything under 10MB anyway
Step 5. Change the extension
In order to open the downloaded file in Quicktime and other video programs, you will need to change the extension. Click the file, wait a moment and click it again. Now you can change the file name. Change the name (the part before the extension) to something else to make it easier to find later. Then add the extension '.mp4' at the end. A window will open asking you to confirm the change. Click 'Yes'. Double click on the file to play it.