Google Gmail

HOWTO: Archiving
Archiving lets you tidy up your inbox by moving messages from your inbox into your All Mail label, so you don't have to delete anything. It's like moving something into a filing cabinet for safekeeping, rather than putting it in the trash can.

Any message you've archived can be found by clicking the "All Mail" label on the left side of your Gmail page. You can also find a message you've archived by clicking on any other labels you've applied to it, or by searching for it.

https://support.google.com/mail/answer/6576?hl=en

HOWTO: Labels
Labels help you organize your messages into categories -- work, family, to do, read later, jokes, recipes, any category you want. Labels do all the work that folders do, but with an added bonus: you can add more than one to a message.

Only you can see your labels, so whether you mark a message with "Best friend" or "Read later," the sender will never know.

https://support.google.com/mail/answer/118708?hl=en

HOWTO: Inbox Tabs and Categories
Your messages are classified into categories such as Promotions, Social and Updates. You can choose to use categories as inbox tabs, and as labels. These categories make it easy to focus on messages that are important to you and read messages of the same type all at once.

https://support.google.com/mail/answer/3055016?hl=en