SQL query syntax question

I am looking at the following query and I have a couple of questions about it.

I understand basic SQL syntax but I am not understanding some parts of the following query.

<cfquery name="GetPossibleUsers" datasource="bc">

                    SELECT   uid, fname, lname

                    FROM     UserInfo UI

                    WHERE    uid IN (SELECT uid

                                                       FROM   UserCampDates

                                                       WHERE  CampDateID = #q_CampID#)

                    ORDER BY lname ASC

</cfquery>

1. Where it says "FROM     UserInfo UI" I understand that "FROM UserInfo"  would mean from the UserInfo table but I don't get the "UI" part. What would the UI part represent?

2. I presume the part:

WHERE    uid IN (SELECT uid

      FROM   UserCampDates

      WHERE  CampDateID = #q_CampID#)

is a kind of compound query (if that is the right terminology) This seems pretty useful but I have not used this method. What would this be called?