create and drop user

Hello

could you explain me (Postgres 6.3.2):

/home/przemol> psql store3

store3=> create user pos;
ERROR: defineUser: user "pos" has already been created
store3=> drop user pos;
NOTICE: Dropping database store3
ERROR: destroydb cannot be executed on an open database
store3=>

I don't understand it :((

przemol