Downloading videos from YouTube has long been seen as a bit of a grey area, mostly because YouTube itself doesn't allow users to download videos directly from the website.However, if you're downloading your favorite videos for personal use, you're not infringing on anyone's copyright.
The main reason for which YouTube frowns upon users who download videos from their website is the fact that the tech giant owned by Google makes money from the advertising they show when you're watching videos online.If you download the videos, you won't be served any ads, so this is why Google doesn't condone the practice.
It is not illegal to download YouTube videos for personal use. Avoiding advertising is not illegal at all, and as such, saving a YouTube video you only want to watch when you don't have an Internet connection is fine when it comes to copyright.
Of course, you are not supposed to share the content online on other websites, or transform it in any way and then pass it on as your own. That would be copyright infringement. But if all you do is watch your favorite videos offline on those situations you find yourself without an Internet connection, that's perfectly legal.
The easiest way to download a video from YouTube is by using an app such as MP3Studio. This way, you can easily download the videos you want with a couple of clicks and you also get the choose the format you want for your video.