Well for the most part an index is supposed to speed querying the table up. An index stores pointers to rowids. So if you have a where clause that uses a field that is indexed, it can find the match in the index and then use the pointer to fetch the rows.
Whereas if there is no index, a query will have to do a table scan, meaning it will go through each and every record and check to see if each record matches the criteria.
Hope this is close.
Mark
