Single Sign On (SSO) one of the most important feature to connect to external systems from SharePoint, In SharePoint 2007 You have to configure SSO service and set the required username/password to connect to external systems.
In SharePoint 2010 the new name for SSO is SSS: Secure Store Service.
Here is some general consideration about SSS in SharePoint 2010:
1) SSS is a service in SharePoint 2010 that runs on the application server.
2) SSS provides a DB that store credentials for ApplicationIDs.
3) Application ID : is your token to access the external application, the token might represents one/group of users.
4) It is recommended to run SSS on separate application pool and on separate application server.
5) Use different SQL Server to store SSS credentials information.
6) You create SS Target Application Types:
a) Group: map group of people to access this external app.
b) Individual: map individuals to access this external app.
This is a general post about SSO - SSS, following posts will explain more....