Every site you set up has certain default permission levels, the most commonly assigned being "Contribute." You have to have Contribute rights to add or edit anything to a list or a document library, so if you have a Practice Group or Community SharePoint, most of your users will have Contribute rights. By default, all "Members" of your site get Contribute rights.
BUT, the Contribute permissions level also allows people to delete things. This really worries me as an administrator. Having to delete something is a relatively rare event, and accidental deletions result in data loss. If it were up to me, all SharePoint sites would be set up to exclude delete rights from the Contribute permissions level. (If a user absolutely needed to delete something, he or she would have to request this, but that little annoyance is more than made up for in better data integrity.)
You can revoke Contributor permission level delete rights in your own SharePoint site, and in my opinion you should. Here's how: