Tribal is a shorthand term used to describe a style of tattoo work that exploded in popularity in the United States and Europe in the early 1990's and which is largely inspired by the traditional tattoo art of indigenous Pacific Islander cultures such as the Dayak of Borneo, and native Hawaiian, Samoan and Polynesian tattooing.
Tribal-style tattoo work is characterized by bold silhouette-style graphics and abstract designs, most often done in solid black. In larger work, tribal-style tattooing is characterized by compositions that flow across the body with the goal of emphasizing the body's natural lines and musculature. It also may include intricate and often repetitive geometric patterns and pictographs that represent flora, fauna, and the natural elements, especially wind, fire, and water. To true native wearers, these designs can have very specialized meanings connoting status, ancestry, the wearer's accomplishments, and tribal affiliations—but, in the highly stylized Western interpretations, these meanings are largely lost.
Tribal-style work has an advantage in that its usually simplified design elements and extensive use of black ink mean that it will hold up very well in the skin, especially for those who tan. Unfortunately, if you don't like the piece later on, it is very hard to cover up with anything other than a larger tribal piece.
Leo Zulueta currently has a studio in Ann Arbor, Michigan called Spiral Tattoo.