When you have a column in which number of distinct values is small compared to number of rows in table, and if this column is used in predicate, bitmap index should be created.
Unlike regular b-tree indexes which stores rowid for each row, Bitmap index just stores a bitmap for each key value. Bitmap indexes should not be used in OLTP tables which are involved in lot many concurrent transactions as they would cause permornance problems.
And what ever question you are putting up are all basic questions, which one should know if working on ORACLE. You better read the concepts guide first at http://tahiti.oracle.com and all your doubts will become clear.
Amar "There is a difference between knowing the path and walking the path."