The difference between RESTRICT and NO ACTION

In discussing foreign key constraints the manual makes the following 
statement about the difference between RESTRICT and NO ACTION.

"(The essential difference between these two choices is that |NO ACTION| 
allows the check to be deferred until later in the transaction, whereas 
|RESTRICT| does not.)"

Can someone explain what this means in practical terms? Do both options 
generate the same error message? Does "until later in the transaction" 
mean that NO ACTION is not checked until an attempt is made to commit 
the transaction?

If someone can point me to a more detailed explanation I would 
appreciate it. Thanks.

Bill