Junk email, also known as spam, is unsolicited email, usually commercial. It can strain networks, clog email servers, and fill mailboxes with unwanted and possibly offensive messages and images. Most of it is annoying, but harmless. Most junk email will be blocked by the email server that hosts your account.
Phishing is a specific kind of junk email that's used to obtain private information for use in identity theft and other scams. The email message appears to come from a trusted source, such as your bank, and often includes the actual business logo and an apparently legitimate reply address.
"Graymail" is generally characterized by newsletters, sales pitches, and stuff trying to pass as legitimate marketing. It's a result of your address(es) getting on marketing lists, and then being sold to other email marketers. These messages are particularly difficult to get classified as spam because they are usually being sent on behalf of otherwise legitimate companies (for varying degrees of "legitimate"), and there are many recipients who consider the messages completely legitimate.
'Backscatter' is the name given to bounce back messages generated when a spammer uses your mail address in the 'From:' line of their messages. This does not mean they have access to your account, however, if you feel your account has been compromised, please change your password. If the spammer's message can't be delivered for any reason, the receiving host will send back a bounce or non-delivery report to the address in the 'From:' line.
If a spammer sends a large number of messages, you may receive literally hundreds or thousands of 'backscatter' messages.
When a spammer uses your address as the "From" address, but they are not sending from your account, this is called 'Spoofing.' This means that they are just using your address so it appears that you sent the message, though the header information will often display the true sending address. For more information about 'Spoofing', please review this article: https://lifehacker.com/how-spammers-spoof-your-email-address-and-how-to-prote-1579478914
There are a couple of options for dealing with legitimate spam in your "Junk Email" folder:
If a message in your Junk Email folder is one that you want to keep, you will be able to mark the item as not junk and the item will be moved to your inbox.
The only way to not get spam is to make sure that spammers do not know your e-mail address or make them think that your account is not being read. There are many ways that you can try to limit the amount of spam you receive.