Well, first, these have nothing specifically to do with the WDTV Live players. These are home network routers, and acts like any other wired/wireless router would, so I’m not sure what your are confused about since what they are is explained pretty much on the product page and being called a “router” should be farily self-expanatory.
Both the N700 & N900 have Gigabit ports to allow for faster network speeds. The N700 is a dual band wireless router that has up to 300+450 Mbps wireless speeds and the N900 is a dual band wireless router that has up 450+450 Mbps wireless speeds.
The only difference is that there is also a N900 Central, which is a N900 with an internal HDD, so you can use it as a NAS. The N900C also has the abiltiy to be accessed from anywhere via http/ftp or by enabling the Remote Access which uses WD2GO.