The official website of youtube doesn’t support downloading their videos. So you have to employ third party applications to download the youtube videos that are actually being viewed as flash content on the youtube.com website.
In this tutorial i will show you how to download youtube videos without the use of any software installation

Follow these steps
1.    Open www.youtube.com and go to the video page which you want to download.
 

2.    Now we have reached the video page. Go to the address bar on the top and copy the URL of the video page on youtube.
 
3.    Go to www.ssyoutube.com by typing the address on the address bar.
 

4.    Once you reach the website. You will find that the website is asking to enter youtube video URL.

5.    Enter the URL of the video page that you copied from youtube’s website, and click “download”
 



7.    After that a list of download links will appear depending upon the availability of the format of the video. Right-click on the format you want and select "Save As..." to start your download.
 
Currently their website support downloads in formats such as MP4, WEBM, etc.