It’s not always simple.
What we have here is a can full of worms. I kid you not.
What we’re talking about is whether uppercase1 letters are treated as different letters from their lowercase counterparts. Is “LEO” the same as “leo” in an email address, URL, etc.?
The answers to the four questions above are: no, mostly no, it depends, and always.
It’s enough to drive you CrAzY.
Upper or lowercase?
Capitalization rules are inconsistent. Email addresses and most web addresses ignore case. Filenames depend on the system, so stick with lowercase to stay safe. Passwords always care about case, so type them exactly. And in writing, proper capitalization matters more than you might think.
Email addresses: no
Email addresses are not case-sensitive.2
These are all the same email address:
- leo@askleo.com
- LEO@askleo.com
- leo@ASKLEO.com
- LeO@AsKlEo.Com
The convention, or normal pattern, is to use all lowercase, so usually you’d see “leo@askleo.com”. Lowercase is such a strong convention for email addresses that using uppercase will likely get you funny looks.
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URLs: mostly no
URLs — web addresses beginning with http or https3 — are mostly case-insensitive.
These are all the same website:
- https://askleo.com
- HTTPS://askleo.com
- https://ASKLEO.COM
- HtTpS://asKlEo.cOM
Once again, the convention is to use all lowercase, so “https://askleo.com” is what you typically see. Since URLs are interpreted in many places, like word processors that automatically turn them into hyperlinks, interpreting mixed-case URLs can be a little dicey. My recommendation is to always stick with all lowercase.
Except…
This is where the “mostly” comes into play. We can make no assumptions about anything that follows a “?” in a URL. For example:
- https://askleo.com?something=something
- https://askleo.com?something=SOMETHING
- https://askleo.com?SOMETHING=SOMETHING
These could all be the same thing, two different things, or three different things. We have no real way of knowing. If you’re ever in a situation where you’re typing something like this in, make sure to type the part after the “?” in the same case as you’re instructed to. (No instructions? Assume lowercase.)
Filenames: it depends
File systems — things like FAT32, NTFS, and others — determine whether the capitalization of a filename on disk matters. Historically:
- File systems that originated in Windows are case-insensitive. (FAT, FAT32, NTFS, etc)
- File systems that originated elsewhere, such as on Apple or Linux devices, are case-sensitive. (APFS, ext4, etc.)
But it gets more complicated than that, especially when uploading or downloading files.
Since so many (perhaps most) web servers are Linux-based these days, case matters. So if you’re called to upload a file, then on those systems, “filename.docx” and “Filename.docx” can result in two separate files, even though they only differ in one letter’s capitalization.
Similarly, say you have those two separate files on a web service. If you download them both into Windows, you’ll get a warning that you’re about to overwrite the first with the second. Windows sees those as the same filename. That the first letter differs in case makes no difference.
My very strong recommendation: in filenames, always use all lowercase if you can. It removes that confusion from the equation, no matter.4
For a little more confusion, sometimes third-party tools like Cryptomator can be case-sensitive even when working on Windows native file systems that are not case-sensitive.
Testing filename case sensitivity
Fortunately, it’s an easy thing to test: create filename.docx and try to copy or save FILENAME.docx to the same location.
- If it works and you end up with two files, then the file system is case-sensitive; they’re two different files.
- If it complains that you’re about to overwrite the first with the second, then the file system is not case-sensitive; the files are treated as the same filename.
- If it works and you get only a single file, presumably with the contents of the second file, then the file system is not case-sensitive; the two files are treated as the same. The filename at the destination may or may not be updated to match the name you copied in.
Passwords: always
Anyone who has had their CAPS LOCK key stuck while entering a password knows this: passwords are always case sensitive. “Password” and “password” are two different (and terrible) passwords.
Written English
Because I make my living as a writer these days, and I’m ever so slightly a pedant, I have to include this.
Written English is case sensitive. There are many rules about what should be used when, but by and large, capitalization matters. People may still be able to read what you’re saying, but capitalization, the lack thereof, or the over-use thereof, sends subtle or not-so-subtle messages about your tone, education level, and even your intelligence. I know we’d like that not to be the case, but it is what it is.
Of course, it’s context-dependent. Text messages are definitely more informal and freeform. Written documents, not so much. Whether email should be stylistically correct is controversial and something that I’m not going to dive into… other than to say that if you want your email to be taken seriously, I recommend you write it seriously.
Bonus: CamelCase
Along with uppercase and lowercase, you may have run across the term camelcase or camel case. In this style, the first letter of each word in a string without spaces is capitalized. The capital letters look like humps on a camel. For example:
CorgiPicnicPhotos
In violation of my “stick with lowercase for filenames” suggestion, this is a common technique to make multi-word folders and filenames easier to read while avoiding spaces in filenames5.
Do this
When in doubt, and with the exceptions noted above, it’s best to stick with lowercase in email addresses, URLs, filenames, and so on.
And of course, please write English properly if you can. 
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Footnotes & References
1: This term harks back to typesetting, where capital letters like A through Z were kept in the upper of two cases, and the others — a through z — came from the lower case. See: Letter case on Wikipedia.
2: At least, they’re not supposed to be. I have vague recollections of mail systems that misbehaved in this regard.
3: Technically, URLs can start with other things, like “mailto:”, “telnet:” and others, but we’re focusing on web addresses.
4: Similarly, I strongly avoid using spaces within filenames because they can cause confusion when being uploaded, downloaded, or copied.
5: See previous footnote.
One application that had case insensitive passwords, that is of personal historical interest, was Microsoft Money.
LOL. Those were the days.
Two services I use are case sensitive for email addresses (e.g. account names.) They don’t care how you capitalize it when you sign in but every log in has to match the original.
The ASCII hex code for the character A is 0041. For the character a, it’s 0061. Those are two completely different characters. In order for programs to treat them as the same letter, they need an arithmetic conversion routine.