Not all equipment slots are created equal! Some items are crafted others are obtained from quests. Some slots can be reinforced for one thing, while other slots can only be energized for another. Knowing what's available can help you plan your best gear for the fastest progression.
The following is a list of each equipment slot with any unique upgrade opportunities for that equip. Also check out the page on gear tiers, as that will also have information on tier-specific bonuses.