Tubemate is a good app, there are many other applications in the internet by which you can download a video fromYoutube but Tubemate is very easy to use, I have been using this for more than 2 years and never faced any problem while using this.
You can download and stream any video of any quality=Even Ultra High definition videos=
The best part of this product is that it is available forFree, You need not to pay even a single rupee for this product.