for example, your PC USB ports are host ports. You can connect devices to it and the OS will see the device and use it.
The port in your phone/tablet is usually a client. You only plug it so that the PC can see it.
Having said that, Galaxy Tab 10" has a USB host port function, and you can plug a mass storage to it, a mouse, or a keyboard, I think.
Some of the cheap tablets even allow you to plug a USB 3G dongle modem to it.
Now, whether your GT-I9003 has host port function, I don't know.