network? shared? ??

I've read some threads that talk about this, but I just don't understand. It may be a very basic thing but I'm new to Macs and I just don't get it. I'm on Lion, Macbook Pro. Can someone explain this to me in basic terms or point me in the direction of support documentation?

What is the Shared list that appears sometimes in the sidebar of my Finder? Who are those other computers off-and-on listed there? Are they connected to my computer or my wireless account or something? I assume that if they're apparent to me, I'm apparent to them -- I don't like the sound of that, so is there some way I can make myself invisible to them?

What is my "network"? Today and once before I couldn't connect to the internet and the error message read something about another "device" on my "network" having the same IP address as me, and I should contact my "network provider." What do they mean by "device"? What is my "network"? (I have not connected this computer with other computers or "devices" in my household. Is "network" my internet service, and my "network provider" my ISP?) My internet settings are set to DHCP, which someone told me in the past means this problem about two devices having the same IP address shouldn't be happening -- so what is going on??

Are these two issues related?

Thank you!

 

1. What is the "Network that my Mac is in"?

2. There are other computers/devices that get an IP in my house, but when I first got that message about another device using my IP, none of them were on (unless an iPod gets an IP? Not an iPhone, just a plain iPod). There are others that belong to my neighbours in the downstairs apartment, but they have their own wireless account. I haven't set up sharing on any of them.

 

I didn't set anything up myself, but I've never been able to access it (nor have any friends who've come over with their smartphones/laptops) without punching in the WEP, so I assumed it couldn't be accessed without that. Am I wrong?

 

@ScoobyMcDoo Thanks a lot, I've taken your advice. Indeed, my wireless admin panel did show a device that isn't known to me; after I set a WPA password, the device was switched from "active" to "inactive" -- I guess someone was hacking into my wireless network after all!

But I'm still wondering about some things -- help from anyone would be appreciated!

1. Why did I twice get error messages about another device using my IP address? All my own devices are set to DHCP (according to my wireless admin panel, and the internet settings on the devices themselves), as is the ex-hacker's device (according to my wireless admin panel) -- shouldn't that preclude doubled-up IP addresses?

2. Am I right in thinking that anytime something appears under the Shared list in my Finder window, it's connected to my wireless network?

3. Would the hacker's connecting to my wireless network cause slower internet speeds and/or down internet services for me, or is that a separate problem? (Many times over the last couple months my internet has been VERY slow and sometimes even non-functional.)

4. When I connect to a public wireless network, I assume I appear on everyone else's Shared list just like they appear on mine. Is there a way I can hide myself? Is this a security risk -- I mean, can other people connect to my computer this way?

THANKS!!