Replace Lookup - searches through the entire database, ignoring any previous searches.
Add to Lookup - adds the contacts from this search to contacts found during the last search (useful for looking up all people with a title of President, then adding those with a title of Vice President).
Narrow Lookup - finds only contacts who meet the criteria of this search as well as the criteria for the last search (ie search for all contacts whose ID/Status is Prospect, then narrow the search and find only prospects in the state of Wisconsin).
Want to save the list as a group for future use? Create a new group, then right click on any name in the contact list and select Add Contacts to Group. Click on the Group name - and the list is set for the next time.