E-mail aliases can be used to protect your primary account, making it harder for spammers and other people to send you e-mails you don't want. Outlook.com comes with support for up to 10 aliases, which you can change yearly. If you've reached the maximum number of aliases or need to provide one in a hurry, you can also use "+" addressing.
Just like in Gmail, which has had "+" addressing for years, all you have to do is add a "+" at the end of your regular user name, then a word of your choice. If your Outlook.com address is email@example.com, you can use firstname.lastname@example.org as an alias. As Ghacks suggests, one of the benefits of using aliases is the ability to create unique filters for each alias.
One drawback to using "+" aliases is that you can't send e-mails as that alias. Your e-mails will always show your real address. In situations when you need to be able to send e-mail as an alias, it would be better to use one of the 10 traditional aliases allowed in your Outlook.com account.