What cords to use to connect to another screen

All you need to connect a laptop with an external monitor is a standard VGA cable (the same one that connects it to a desktop). These usually have blue connectors.

There's no way (that I know of) to connect one computer to another computer just to use the display attached to it.

If you wish to be able to use a display with the laptop without having to first disconnect it from the other computer every time, you'll need what's known as a KVM switch (Keyboard Video Mouse). I'm using one right now to hook up my ASUS laptop to a 19" widescreen display, but with the press of a button (on the KVM), I can switch to the other computer it's hooked up to. The design also allows the external mouse and keyboard I have hooked up to work with that computer, as well.

You can't access both computers simultaneously this way, but it's great for moving back and forth between them. I use this to check email, Facebook, etc. on one computer while fixing the other.

There's a TON of choices for KVMs. I'm currently using an IOGEAR USB KVM for 2 computers. It's the GCS22U. I can't say whether it's any better or worse than others, but it's come in handy for me. I haven't tried any others, so you might find one that works better.

Basically, the box has input connections for a monitor, keyboard, and mouse (USB, in my case). You hook those devices to it, then take the other cables (on the CGS22U, they're hard-wired to the box) and connect those to the other systems. The box is powered via the USB cables, so it doesn't require external power.

I find KVMs immensely useful and plan to upgrade to a bigger one at some point. They do get expensive as you go up the product lines, but that's beyond the scope of your question.

I hope this helps!