File quarantine-aware applications that download files from the Internet, or receive files from external sources (such as email attachments), attach quarantine attributes.

Quarantine-aware applications include Safari, Messages, iChat and Mail.

These attributes include date, time, and a record of where the file was downloaded from.

When you open a file received through a quarantine-aware application, OS X warns you where the file came from. You receive an alert asking, "Are you sure you want to open it?" You should click Cancel if you have any doubts about its safety.

If you have multiple user accounts on your Mac, the user account that downloaded the file is the only user account that can remove the quarantine attribute on a file. All other user accounts can open a quarantined file, but they are still presented with an alert asking "Are you sure you want to open it?" every time they open the file.