Question 1) If all Ethernet cards are filled can i connect another pc on the usb port?
If the USB port on the router is a print server then no you cannot use it as a NIC. If you are un-familiar with that router it could also be a USB port to add a USB external hard drive to.
Could you port the make andmodel router that you have?
Question 2) For routers we do not run driver programs. In case we attach a USB printer, where the drivers are installed?
You may need drivers to configure the print server but typically you do not need drivers for the router to work. If you enter the router's setup page you will likely find a page on setting up the print server.
Question 3) From the other local pcs how do we send print jobs to the usb printer?
You will setup the printer through the networking rather than locally. There will be a CD that should have come with that router to configure the individual computer. You will need the printer drivers on the computer you are configuring, just like if you were connecting the printer directly to the computer.
Question 4) This usb printer does not have an IP, am i wrong?
You are right. the printer does not have an IP address but the print server does. You will see it when you configure the print server from within the router setup pages.