Cursors and waiting users

Linux kernel : 2.0.36, postgresql-6.4.2.

Hello!

I run psql and begin command with transaction :
xxx=>begin work;
BEGIN
xxx=>declare kur1 cursor for select * from tab_name;

Then I run psql on second session and I try to insert something to this
table :

xxx=>insert into tab_name values ('something');

I can't finish this command until I close transaction on first session.
Why?
Does the declaring of cursor lock table for insert and I can't insert into
table? But I use cursor only for viewing of table.

Than, if one user use cursor on the table, second one can't insert data to
that table?

I am beginner in postgress. Help me find my mistakes.
(Sorry for my English).