XBee Communication

I am just getting started with Arduino and I'm looking in to using it for a project that will most likely require multiple Arduino devices, or ALOT of cable. I was just trying to figure out this whole XBee concept. I understand that it links Arduino devices together to form a mesh network for communication, but I have a few questions that weren't addressed by the basic reading. Do I need to connect the XBee devices to my router? Is it easier for Arduino communication that WiFi? Can one Arduino device be set to control the other two via XBee? Will other wireless signals in my house interfere? Lastly, will the pins required for XBee make it impossible for me to add other shields to the Arduino such as servo shields or RF shields?

Thanks in advance