1. An email address is NOT case-sensitive. This is always true (at least I have never heard of an exception to it). So you can type an email as firstname.lastname@example.org, PCTUTOR@GMAIL.COM, or PcTuToR@GmAiL.cOm (although that last one is pretty annoying) and it will still get through.
2. A website address is not case-sensitive before the “.com”, but it is probably case-sensitive after that point. So if you are only typing “www.yahoo.com” or “www.google.com” it doesn’t matter if you type it in upper or lower case. But if you have a longer URL, such as “www.mydomain.com/FunnyPictures.html” you may have to pay attention to the capital letters. If you are not sure, try all lower-case first and then some other variations if that doesn’t work.
3. A password may or may not be case-sensitive. If you are creating an account and have to come up with a password, some sites require that at least one character be capitalized. If the site does not designate a password policy, it probably doesn’t matter.
4. Captcha verification may or may not be case-sensitive, but it usually is not. This is that “we must verify that you are a human” test in which you have to type a word that is barely legible in order to submit a form on a website:
Yes, they are annoying and they slow down the whole ordering process for a lot of sites, but they are unfortunately necessary due to the large volume of scammers in the world. But they are not usually case sensitive. Even if you get it wrong the first time, you can try again.