A tablespace is a logical space allocated on the harddisk to create tables in (do not confuse it with datafiles, more than one tablespace may exist on a datafile and a tablespace may consist of more than one datafile). you can give it a number of storage parameters. Every table created within the tablespace will by default have the same storage parameters unless defined otherwise.
It is good practice to create a number of different tablespaces: one or more for business data, one or more for indexes, one or more for temporary data (Oracle has a standard temporary tablespace (temp_segment) and needs this when executing large sor and join operations) and one or more for rollback (rollback_segment).
There is a lot more to tell about tablespaces, I find the oracle documentation quite readable.
Martijn Tromm