need help setting up a very simple home network configuration

I'm probably asking something has been asked before but since I don't know anything about networking I don't even know the right buzz words to do a good forum search.

I have what I think is a very simple network need, like this:

network diagram.jpg

One person told me the box with the question mark just needs to be a switch, another that it needed to be a "firewall router", and another that it needed to be BOTH a switch and a firewall router, and another that it needed to be "managed switch".

I've looked all those up but being totally new to networking I find it all very confusing.

Any suggestions?

Thanks

Sorry, I should have been more clear about what it is that I am really trying to do. I want the laptop and desktop to be able to see each others hard drives, or at least one of them each for ease of file transfer, but I don't want the guest computers to be able to see any of that.

Thanks. I'll check out the links. So I take it a switch is either a bad idea or just not needed?

As far as I know, the ISP only provides a single IP address, and yes, I want my network to be able to access the guest network but not the other way 'round, although that's not crucial. Big thing is I don't want the guest network to be able to access anything on my network.