"UD" stands for "Ultra Durable" marking.
This marking are mainly used only by Gigabyte and it means the level of quality and durability of the components that were used during the assembling of that particular mother board.
Simply putting: the higher the "UD" number on the Gigabyte's mother board, the higher it's overall quality and durability. "Good quality" Gigabyte boards start from at least "UD 3" level.
"High quality" Gigabyte boards (top of the mainstream, and highest point of middle-price segment) are marked with at least "UD 5" level.
Top quality Gigabyte boards (enthusiast and extreme OverClocking, not meant to be used by a typical mainstream user, high-price segment) start from "UD 7" level, and go up to"UD 9".
UD 3 - Beginner Level (Better not use it if you're going to OverClock higher than 3.8GHz. Still quite good for single-pair SLI/CFX, tough).
UD 5 - "High quality for a reasonable price" Level (If you're going to OverClock to at least 4GHz, but not higher than 4.4GHz, this the board for you. Still only single-pair SLI/CFX).
UD 7 - "Highly Experienced" Level (Good for those who definitely plan to OverClock to at least 4.4 GHz, but not higher than 4.8 GHz. Pretty much capable of 3-way SLI/CFX).
UD 9 - GOD TIER Level (also known as "SHUT UP AND TAKE MY MONEY!" Level >: ). Can do everything with ease: 5+GHz OverClocking, 4-way SLI/CFX, and many MANY more).
Master Chen, thank you so much for taking the time to answer what i'm sure seems like a very rudimentary question, but i am replacing the motherboard in my gaming computer that is currently a GIGABYTE GA-P55M-UD2 and having alot of trouble finding one. can you recommend a board that is as good or maybe a little better? please keep in mind that i have the skill level of a novice in this endeavor, and the main reason i was looking for the same kind of board is that i was told that it was the easiest route since i would not have to reprogram anything. again thanks in advance for any advice you can give.
Can you recommend a board that is as good or maybe a little better?
It depends on what you mean by "I was told that it was the easiest route since I would not have to reprogram anything".
When you change your motherboard for a newer one, you will have to re-install your Operating System and most of the software that you use (except for "Portable" software and applications), because Operating System and the software that you installed while using your old motherboard, won't work anymore.
Because when you install something new on the motherboard, most of the software (and Operating System especially) are given special assignment code that is written into your motherboard's memory, simply putting - you won't be able to use that software on another motherboard unless you install it again from the scratch because your previously installed software got strictly assigned to this particular motherboard of yours. But, again, this doesn't apply to the "Portable" versions of software, because "Portable" software (by the most part) doesn't even have to be installed (and thus didn't get any assignment code written inside your motherboard) anywhere, it can be used right away.
Even if the two separate motherboards are two absolutely same models, they still have different ID marks written in them, and thus you can't use on another motherboard your Operating System or the software that is stored on your HDD, even if they are exactly the same, until you completely re-install that software for that another motherboard.
Aside from that:
If you want to upgrade from "P55 UD2" to something better, the simple solution would be to get a higher UD model on a better chipset.
Otherwise, I highly recommend either this or this (if you can afford it).
But if (for some reason) you don't want to change the chipset, then yeah - the Gigabyte's "GA-P55M-UD4" would be just fine (if you can find it, that is).
Remember that if you do decide to change for a motherboard with a better chipset, you would have to also change the processor, because 1155 socket and 1156 socket are different, and their processors are not compatible with each other one sockets.