Mulit-User Locking??

Hi All:
I got a question right now, please let me a hand.

I am wondering is Postgresql will do something like session on the back
end that will lock up a table or database while a table or database is
accessed by one user. Then all other user cannot access that table or
database.

As I know, there is one for the sybase. I have to put a line something
like:
"read commit isolation"
at the end of the SELECT SQL statement to let all other user to access
the table or database during I am SELECT (only browsing) the table.

Please, let me know either there is a feature like that for the
Postgresql.
THANK YOU VERY MUCH.

Hary Yau