First... ArkInventory rules/pwns/is awesome etc. ;) Aah... I just had to get that one out there, now on to the "rules" part :)
ArkInventory is a a bag replacement for World of Warcraft which gives you a massive amount of options to sort and order the contents of your bags and bank. The way ArkInventory does this is by splitting the bags into sections according to the users desire. For example have a section with all your tradeskill items, another for all your trash etc.
For most people ArkInventory will do the job just fine by using its categories. Using its rules makes it even greater. With rules it is possible to specify multiple criteria for how the sections should handle items. Here are my favorite rules.