Looted items are equipment or vehicles that Orks have appropriated and refurbished. They acquire this technology either as battlefield salvage or outright theft. Since other races' tech isn't very orky, the Orks make it so, welding pointy metal bits and pieces onto it.
This used to be represented on the tabletop by allowing you to purchase vehicles from the Space Marines and Imperial Guard codices to use in your ork army as long as you made the vehicle look like it had been modified by orks. This wasn't just a principle thing; the rulebook literally said that you had to make the vehicle look orky, so no Proxies allowed. However, the vehicle always had to make use of the Orks' Ballistic Skill 2 - no matter what codex the vehicle came from - at exactly the same price, so unless you had good luck, an ork-looted vehicle would lose to its non-looted equivalent in a firefight. You also had to deal with the risk of the vehicle breaking down at the beginning of the game on a D6 roll of 1.
Suffice to say, this was a ridiculously popular option, lowered the barrier of entry for army newcomers (because they could convert existing vehicles, leading to a lot of Imperial Guard or Marine players gradually building some token Ork forces), and made it way easier for Ork players to cover holes in the army they'd otherwise have, such as a general lack of heavy armor and stronger firepower - taking a Leman Russ Battle Tank or two was a good way to get heavy armor and firepower on the battlefield. It was so popular, in fact, that Dawn of War made Looted Leman Russes the dedicated Ork heavy vehicle, and Dawn of War 2 gave them looted Predator Tanks.
Unfortunately, like anything remotely popular in Warhammer 40,000's playerbase, it was inevitably ruined by GeeDubs. This particular rule change was especially loathed by players, as the very codex update that heralded this change (5th Edition) prominently featured a looted Basilisk in the Cityfight showcase in the same book. The upcoming ork Codex would remove the Looted vehicle rules entirely, with the only surviving example of it being the Looted Wagon, an overglorified Rhino with a more forgiving rule to represent it not being made by orks.
In the case of other vehicles, you would either have to have them count as a different vehicle in the ork codex, or more commonly be considered an allied unit, which opens up its own can of worms because the Ork ally list features only Chaos as a remotely viable ally - good news for that one maniac who made an orky Defiler; terrible news for pretty much everyone else.
Looted Ghost Ark
WE'Z GOT DA BEST BOX ON DA FIELD
|Forces ov da Orks|
|Bosses:||Big Mek - Meganobz - Painboy - Warboss - Warlord - Weirdboy|
|Boyz:||Boyz - Gretchin - Nobz|
|Oddboys:|| Burna Boyz - Flash Gitz - Kommandos |
Lootas - Mekboyz - Stormboyz - Tankbustas
|Stompy 'fings:||Deff Dred - Gorkanaut - Killa Kan - Mega-Dread - Morkanaut|
|Transports an' Tanks:|| Battlewagon - Big Trakks - Bonebreaka - Bonecruncha - Braincrusha |
Flakkatrakks - Grot Tanks - Gutrippa - Grot MegaTank - Gunwagon
Looted Wagon - Trukk - Spleenrippa - Weirdboy Tower
|Speed Freaks:||Junka - Warbikers - Warbuggies - Wartrakks|
|Flyboyz:|| Deffkoptas - Bomma - Dakkajet - Fighta - Fighta-Bomma |
Minelayer - Warkoptas - Wazbom Blastajet
|Supportin' Dakka:||Grot Bomm Launcha - Mek Gunz - Squig Catapult|
|Splashy Noggins:||Ork Submersible|
|Zoggin' Big and Ded Killy:||Battlefortress - Gargant - Kill Tanks - Stompa - Squiggoth|
|Gubbinz an' Wots-its:||Choppas - Fungus - Ork Gunz - Snotlings - Squigs|