youtube-dl and ytdl-core

This is a short guide aimed at beginners to explain what youtube-dl and ytdl-core are and how to use them effectively.

Preface: are you a programmer?

youtube-dl and ytdl-core are not user-friendly websites or GUI applications for downloading YouTube videos!
The projects are meant for embedding in computer programs.
If you're looking for a YouTube downloading website, check out safedl; it uses ytdl-core under the hood.
If you're looking for a CLI of each library, here's youtube-dl's binary instructions, and here's a binary wrapper for ytdl-core.

What are these libraries? Which should I choose?

youtube-dl and ytdl-core are libraries, or a chunk of code that can be used by another computer program.
ytdl-core is written in JS to be used in a Node.js program. It's distributed on npm and developed on GitHub.
youtube-dl is written in Python, to be used from another Python program. It's distributed on pip and developed on GitHub.
If you're new to programming, you should probably learn the basics of Python or JS before continuing.

youtube-dl is older and larger than ytdl-core. It supports many sites and makes a binary (CLI) of it under the same project.
ytdl-core is more exposed than youtube-dl; in other words, you have much more control over what ytdl-core actually does.
You should use the one that best fits your project's needs, i.e. if one has a feature you need or is in the language you'll code in.
Keep in mind that most languages can utilize binary files, so if you code in another language you can still use the bins.

Getting started with youtube-dl in Python

Quick note: Python 2 and Python 3 behave differently; I'll only teach usage in Python 3.
We've finally gotten past the boring part. Glad you're still here.
Start by installing the library in pip. You'll need to do this from the command line.
How exactly this happens varies a bit by machine, but it's probably pip install youtube_dl. In some cases it might be pip3 install youtube_dl.
Make sure you keep it updated!
Basic usage of youtube-dl in Python is as simple as it gets:

import youtube_dl
youtube_dl.YoutubeDL({}).download('videourl')

This code will download the video at 'videourl' directly to disk. Let's talk a bit about exactly what that code does.
The first line, import youtube_dl, looks for the module you just installed with pip and makes it usable in the rest of the file.
You only need this line once in a file.
The second line's a bit complicated, so let's break it down.
youtube_dl.YoutubeDL references the class called YoutubeDL from the import youtube_dl statement you wrote before it.
({}) provides blank (default) options to start the class. This is required.
You can provide any other options by putting them inside the {}, for example {'quiet':'true'}.
Finally, .download('videourl') uses the YoutubeDL class that's before it to download 'videourl' using the options in {}.
If you're going to do multiple operations from the same instance of the YoutubeDL class, a with..as statement can make it easier:

import youtube_dl
with youtube_dl.YoutubeDL({}) as ydl:
    ydl.download('videourl')
    ydl.download('videourl')
    # etc...

TO learn more, check out youtube-dl's documentation to learn about all the things you can do from the Python module.

Getting started with ytdl-core in Node.js

Start by installing ytdl-core into your project's JSON file: npm i -S ytdl-core@latest. Make sure you keep it updated!
Using ytdl-core in Node.js is also a cinch:

const ytdl = require('ytdl-core');
ytdl('videolink');

Let's break this one down as well.
The first line, const ytdl = require('ytdl-core');, has the same function as the import statement from the Python program: it takes the module from package.json and makes it available in the file. Again, you only need this line once per file.
Then ytdl('videolink') returns a Stream, a special type of data in JS that streams from one source to another.
You can use pipes, listeners, or functions that take streams to use this data in the next part of your program.
Another function that ytdl-core has is ytdl.getInfo('videolink').
It's async, so you'll have to wait for it to finish, for example with await.
When it returns, you get an Object with all of the video metadata, formats, etc.
It's quite a helpful function, and makes it my downloading library of choice.
Check out ytdl-core's GitHub repo for more detail and other functions that you can use with ytdl-core.

Footnote: Did I miss something?

Please comment with ideas to improve this Gist.

redbrain/An overview of youtube-dl and ytdl-core.md