"Email spam," "Junk email" or "Spam mail", usually referred to as just "Spam", are unwanted, unsolicited, commercial email messages, normally sent in bulk to a large distribution list of recipients.
Many spam emails contain URLs to websites and can be annoying as well as potentially risky because they can contain bogus offers that can cost you time and/or money.
In Office 365, your spam messages are placed in quarantine. You have the ability to review and release messages from quarantine into your inbox.
Clutter is a mailbox management feature available to you that can be turned on or off.
Clutter differs from "junk" email because it comes from valid sources or people that you have interacted with before. The message is not unsolicited (like junk), but it may be low priority to you. So the Clutter folder is basically your second-tier Inbox.
If you receive a lot of emails a day, the Clutter function might be a real time-saver long-term, since Clutter gets smarter as you use it.