That's a very good question. It's certainly against YouTube's terms and conditions, which say you can only use its content for streaming, and you could argue that it's unethical too – many artists depend on YouTube's ads for their income, so any ad-free playback you do is depriving them of real cash money.
We suspect the legality of this is similar to ad-blocking: content owners would really rather you didn't do it and try their best to stop the tools from working, but there's not a lot they can actually do to prevent you watching videos ad-free if your listening comes under 'fair use' in copyright terms. Then again we're not lawyers, so proceed with this at your own risk.
What formats can I download YouTube videos in?
Video downloading apps usually give you a choice of file formats. The most common are MP4, 3GP, AVI and MOV; the choice of format and quality settings will affect what you can watch your video on, and how good the video will be.
If you're not too fussed about video quality, 3GP is the file format for you. It's perfect for small-screened mobile phones and awful on anything else, but the files are tiny – which matters on old mobiles, as they don't usually have very much on-board storage or any memory expansion capabilities.
For other devices, MOV is Apple's video format and works on every Mac, AVI was developed by Microsoft and works happily on PCs, and MP4 is the video equivalent of MP3 and works on pretty much anything. For maximum compatibility we'd suggest sticking with MP4.
How about video quality?
The next decision you'll need to make is the video quality. If you're downloading via a service such as ClipConverter – a process explained on the previous page of this article, you'll be given a choice of video sources. That's because YouTube streams in multiple qualities ranging from 3GP for old mobiles to 1080p high definition for big HD displays.
3GP aside, you have four options here for your computer, smartphone or tablet: 360p, 480p, 720p and 1080p. The higher the quality the bigger the file, so the trick is to balance quality and size: unless you're watching on a really big screen, you can probably manage without 1080p quality unless you've got oodles of spare storage (although 360p video won't look good on an HD display).
Unless you have unlimited storage capacity, the trade-off usually means finding the lowest video quality that you can tolerate on the display you'll be watching the video on.
If the video you're downloading has music, it doesn't matter what video quality you choose – the soundtrack has the same bitrate whether you go for a 480p video or a 1080p one. That's because YouTube's audio and video streams are two separate things, so adjusting the video quality doesn't make any difference to the quality of the soundtrack.