Originally, there were two types of Wagner tubas, (tenor and bass) though most modern players prefer a "double tuba" that can play in either range with the flick of a valve. Either way, the the Wagner tuba looks like a small oval tuba, or a slightly elongated French horn. It is played in the lap with the bell facing up. The instrument was designed to have a narrow pipe bore (like the horn) so it can be played with a conical horn mouthpiece. Also, like the horn, the Wagner tuba has a rotary valve system, and it is notoriously difficult to keep in tune.
During performances of the Ring, four members of the eight-man horn section are required to put down their horns and pick up the Wagner tubas (usually two tenors, two basses.) This has the advantage of adding a darker quality to the orchestra, and a clear, stentorian sound in the more noble passages. A good place to hear the Wagner tubas do their stuff is in the opening of Act I of Die Walküre, where they represent the storm scene and the German thunder god, Donner.
Although Wagner only used these tubas in the Ring, they also appear in symphonies by Anton Bruckner (most notaly the Seventh, which was dedicated to Wagner) and Schoenberg's mighty Gurrelieder. Stravinsky required them for the Rite of Spring. Finally, Richard Strauss, who never met an orchestral texture he didn't like, used them in a number of his works.
In Germany, the Wagner tuba is also known as the "Ring-Tuben", the "Bayreuth-Tuben" and the "Rheingold-Tuben."
