Many people choose to get a skull tattoo because they want to show their affiliation to a group or to show their beliefs and their devotion to them.
The famous Jolly Roger (the black flag with a white skull and two human bones crossed under the skull) once used by pirates, was both an intimidating signal and a symbol of the total disregard for the authorities, as well as a symbol of the absolute freedom. This symbol (which is still preserving its original meaning) symbol was integrated into various systems of symbols belonging to different subculture or/and counterculture groups (like the outlaw motorcycle clubs whose members often get Jolly Roger tattoos to show their lawlessness). Sometimes, the anarchists use the "Jolly Roger" as a symbol of rebellion against the organized authority.
The Dead Man's Head consists usually of the human skull with or without the mandible and often includes two crossed long-bones (the femurs). Unlike the Jolly Roger design, the Dead Man's Head design features a skull with the crossbones placed behind the skull, instead of beneath it. The Dead Man's Head is an ancient symbol and has had different meanings over time; in the Elizabethan England, for instance, The Death's-Head Skull (usually a depiction without the lower jawbone), was emblematic of bawds, rakes, sexual adventurers and prostitutes; the term Deaths-Head was actually parlance for these rakes, and most of them wore half-skull rings to advertise their station, either professionally or otherwise. The original rings were wide silver objects, with a half-skull decoration not much wider than the rest of the band; this allowed it to be rotated around the finger to hide the skull in polite company, and to reposition it in the presence of likely conquests.
The Totenkopf was used by the german Schutzstaffel (SS) as the unit's insignia. As a consequence of that, this particular design of the Dead Man's Head has become closely associated with various movements like the "white nationalist" movements, the "white power skinhead" movements and the "Neo-Nazi" movement.
Totenkopf design; the draw also contain a variant of so-called "Cross Pattee (English: Cross Patty; German: Tatzenkreuz)"; the Cross Pattee is often mistakenly identified with the gothic crosses, but actually it is an ancient symbol which was largely used by the Teutonic Knights and sometimes by the Templiers. The Cross Pattee design was incorporated in the design of the "Iron Cross", a german decoration for military valor established by the King Frederick William III of Prussia in 1813. The Iron Cross remained in use, in various forms, by Prussia and later Germany until 1945.
Variations on the Skull and Crossbones are used by various military units as a symbol of strength, courage and bravure, as well as a symbol of the disregard for one's life. On wikipedia you can find a list of the Skull and crossbones military insignias.
The BOPE emblem, a skull impaled on a sword, backed by two gold pistols sends out a strong, unambiguous message. The emblem symbolises armed combat, war and death. The BOPE’s emblem, a skull with a crisscross of pistols, is believed to possess mystical powers of protection. BOPE = Special Policial Operations Battalion, a special forces unit of the Military Police of Rio de Janeiro State, Brazil. If you want to know more about BOPE you can visit their blog or you can watch these two movies: Elite Squad and Elite Squad: The Enemy Within