Microsoft SharePoint 2010 : Permissions

SharePoint permissions fundamentally define two things: who can see things and who can do things. Permissions can be applied at many different levels, but for ease of management, they are usually defined at the site level. Generally, groups are then used to control which users have rights to each site. This makes permissions for the most common collaboration scenarios relatively easy to manage.
Furthermore, many SharePoint sites, lists, and items are configured to use permissions inheritance. This means that many assets in SharePoint don’t have their own permissions applied—they inherit permissions from their parent. This is not only convenient, it is also a SharePoint best practice. While there may be plenty of reasons to define security at the site level, adding security at the list level—or even the item level—can make your permissions management so complicated that you don’t really know who has rights to what.

To modify the permissions on a site, go to Site Actions | Site Settings | Site Permissions to open the Permissions page .Although it is possible to assign permissions to SharePoint items, it is generally not considered to be a good idea. Once you start using permissions other than at the site level, it becomes vastly more difficult to manage and track the permissions of each user.

In SharePoint 2010, there is a new feature called Check Permissions that allows you to go to the settings of a list and quickly see what rights a particular user or group has to the library. To use the feature, go to the library ribbon and click Check Permissions .
After you enter the name of a user or group, the results will show the relevant rights and whether those rights have been granted directly or as a result of group membership .