Bound items are a feature implemented in MMORPGs to avoid abuse of twinking.
Bound items are designed to prevent them from being shared with other characters, to avoid easily making overpowered lower level characters. If you have a bound item, it can (generally) be sold to in-game vendors, but it cannot be traded to another player.
Unbound items, however, can be freely traded from player to player, and can be used by players who aren't the ones who initially obtained the item (whether from quest, random drop, or by purchasing it from a vendor), assuming that the player meets all of the other requirements for the item.
Bound and unbound is not random. Quest items are typically bound, as they are the reward for that character completing the quest, and therefore it is expected that that character will be the one to use it. The same applies for specific reward drops from difficult encounters, such as raid or group instances.
It's actually sort of confusing. There is a lot of posts out there, including from official SWTOR sites that don't tell the whole story. Most of them suggest that there aren't inherit stats. Which is sort of true, but not totally:
From SWTOR Stats & Attributes:
Due to the extreme amount of gear customization in TOR, developers
want you to be able to take any orange item (orange = fully moddable)
and use it all the way to max level if you prefer. In order to achieve
this, weapons and armor both have a dynamic stat on them which changes
based on the level of the mods you have placed inside the item.
- Rating – Increases the damage done on a weapon. You’ll see this listed
at the very top of a lightsaber or blaster’s tooltip. The stat itself
does absolutely nothing except increase the damage done — think of it
as the ‘item level’ stat you’re used to seeing in games like World of
Warcraft.
- Armor – Armor, which you should be more familiar with,
increases Energy and Kinetic resistances (more on that later), is the
dynamic stat increased on pieces of equipped gear.
So, for an example, if you take an orange lightsaber with all level 1
mods in it, compared to the same lightsaber with all level 10 mods in
it, the Rating stat will be much higher, and thus so will it’s damage.
Same goes for Armor on those pieces with Armor on them.
(Which is a good summary from this really long SWTOR.com forum thread that did have an op post in it that clarified the situation to what we know now.)
The op explained that this dynamic stat is important so that modded items can drop from bosses on instance and still have some inherit value so that the first boss in an instance isn't farmed over and over for only the mod pieces.
The above SWTOR.com thread also discusses one important bug that is confusing a lot of people. If you do not equip the weapon/armor or relog after modifying it, the tooltip will not accurately reflect all the correct stats based on this dynamic stat modifier. Depending on what you change, when, and what is in it now, the numbers in the tooltip can be a lot higher or lower than they really are.
UPDATE:
Some additional clarification from the official devs:
Orange Armor gets it's armor value from the 'armoring' mod you slot
in. If you exchange it for something better, the armor value increases
along with the stats.
It's always the first mod in the list that determines the intrinsic
properties of the orange items (e.g. armoring on armor, barrel on
guns, etc.)
Best Answer
Orange items are items that can be modified with various mods, like lightsaber hilts, armor, augments, crystals, etc.
The order is:
Note that orange is not in that list. Orange just means you can modify an item; it does not indicate a specific rarity level.
Some orange items have base stats included in them, some do not. The whole idea behind the orange items is that if find gear you like, you can keep upgrading the mods to keep upgrading your stats as you level, but allow you to keep that same armor/weapon type.
There are no level caps on mods per item, as long as you have achieved the requisite level for the mods.
UPDATE:
Regarding inherit stats with orange items: We have been granted more knowledge by the SWTOR devs. See the answer at https://gaming.stackexchange.com/a/45457/1134. In short, yes, there are inherit stats to fully-moddable orange items... but not exactly. See that link for the confusing details.
UPDATE 2:
From the official forums: