What is the difference between User Profile in Central Admin and User Information List in a Site Settings?
I want to add some columns and lookups to Users of a SharePoint site in order to access them in Workflows. How can I do that?
The user info list is vastly different from the User Profile. Each Site Collection maintains a list of users. In Foundation that is all you get. In Server the User Info List (UIL) is synchronized with the User Profile through a couple timer jobs based on user site activity.
The user profile is supported by the User Profile Service Appication. The user profile is imported from an LDAP source, usually AD. You can add fields to the User Profile trhrough the Service Application Management interface. Setting the field to replicate will push the field to the UIL in the Site Collection. Bear in mind that this will make the data avaiable to all users, so don't put private information here.
Rather than push to the UIL it is preferable to look up the info from the User Profile Service through the web service (in 2010) or the REST service (2013) from your workflow.