WUBRG is a term used in Magic: The Gathering to describe a card or ability that costs one of each colored Mana: White, blUe, Black, Red and Green. Blue uses the U because Black already used the B and the L could be confused for a Land card. Plus, it's got something of a ring to it. Pronunciation is either as wub-rg or as woo-berg; exactly how is up for grabs. It is also the order in which mono-colored cards are numbered in a set: first all white cards followed by the blue, black, red and green ones.
Anything with the cost of one mana of each color has a cost of WUBRG. It is difficult to be able to cast this using basic lands, given the low chance you have of having all five lands on the field at the same time. As such, workarounds are needed. Creatures that can generate several kinds of Mana are an option, as is Prismatic Omen. An inverted version of this would be Fist of Suns, that allows you to pay WUBRG instead of a card's regular mana cost. As such, WUBRG cards don't see a lot of use in play, although a few of them can make for interesting commanders in a game of Commander.
As of this writing there are twenty-three different cards with a casting cost of WUBRG. A twenty-third would be Progentius with a cost of WWUUBBRRGG, and five of the cards (Coalition Victory, Conflux, Karona, False God, O-Kagachi, Vengeful Kami, and The Ur-Dragon) have colorless mana in there as well. The Reaper King has its cost as two colorless or one colored for each color, allowing its cost to be (10), WUBRG or anything in between. Eighteen of these cards are creatures, three are sorceries and three are enchantments (one of which is an enchantment and a creature).
Cards in the WUBRG theme are often embodiments of all magic, an entire plane, the most powerful Slivers, legendary rulers and so on. The non-creature cards are works of truly epic magic; the Conflux of Alara, the victory over the forces of Phyrexia and the spirit of Kamigawa are all WUBRG to cast. WUBRG cards are only printed rarely and thought out first before being put to cardboard, even such messes like Last Stand.