[Tutor] Understanding DBAPI cursor.execute

[John Fabiani]

I'm using MySQLdb (but I'd like to use others DB too) and I'm trying to 
understand the DBAPI cursor.execute.
when I use
cursor.execute("SELECT * FROM mytest where address = %s",string1)
above works but  
cursor.execute("SELECT * FROM mytest where address = %s" % string1)
above does not work.
So why did 
 cursor.execute("SELECT * FROM mytest where clientID = %d" % numb)
work?
clientID,address was created using
clientID int not null auto_increment primary key
address varchar(40)

Can someone explain why the difference and does the same thing work with other 
DBAPI drivers? 

Thanks for the help - someday soon I hope I'll be able to help.
John