On the site you are able to do several things. The two most exciting options are:
1. Load a character in to the site and check for upgrades to gear. The site will tell you the best upgrades to get for your gear and where they drop. It can also 'Optimize' your gear...changing enchants/gems and reforging gear to get the 'optimal' configuration for your gear set up...very handy if you're not 100% sure on things like reforging (which can be confusing at times)
2. You can pick a class/spec and it will give you the best-in-slot list for that class/spec. Handy if you simply want to know what best-in-slot is for you without worrying about your current gear.
The site also lets you choose what level of gear that it displays, underneath the gear list simply change the options to suit your circumstance. You can and should select your race and professions and if Archaeology and BoE items are available to be selected from.
There are also a few other things on the site that may be of interest to you, have a poke around and see. Overall an excellent site that I myself have found very useful!