The RID Lookup operator offers the same logical functionality within the execution plan as the Key Lookup operator. But where Key Lookup is used for tables that have a clustered index, RID Lookup is instead used when a table is “heap” (table without clustered index). It is used when another operator (usually an Index Seek, sometimes an Index Scan, rarely a combination of two or more of these or other operators) is used to find rows that need to be processed, but the index used does not include all columns needed for the query. The RID Lookup operator is then used to fetch the remaining columns from the heap structure where the table data is stored.
Click through for a great deal of information about RID Lookups.