Question: I just noticed that when people clone a remote repository from Github, they sometimes use a URL starting with 'git://' instead of 'https://'.

I have never heard about the former before.

Is there any noteworthy difference by any mean between those two url conventions?

Is there any other form that is like 'git://'?

Answer: 
These are two different protocols that can be used to communicate with the remote git providers. The major differences being that the `git protocol (git://)` is much faster than `Http Protocol (https:// or http://)` as it uses the **same data-transfer mechanism as the SSH protocol but without the encryption and authentication overhead**.

So generally, you can use SSH or HTTPS access for developers who have push (write) access and have everyone else can use `git://` for `read-only` access.

There are other protocols as well like ssh (kinda fast but with authentication) and local protocol. The detailed pros and cons and be found in the official wiki - <https://git-scm.com/book/en/v2/Git-on-the-Server-The-Protocols>
`git` supports 4 different protocols to connect with; `ssh`, `http`, `https` and lastly `git`. Each have their pros and cons. For example `git` protocol have a dedicated port number (9418) and it is often the fastest protocol available, but comes with no authentication. In contrast, `https` has authentication but it is tricky to set on up on a server.

Perhaps take a look at [this link](https://git-scm.com/book/en/v2/Git-on-the-Server-The-Protocols) for more info.