Human skulls and skeletons are one of the most common elements in the Grim Reaper tattoos, due to the fact that skeletons and skulls have long been associated with death and decay (sometimes Death himself is portrayed as a skeleton).
Skull tattoos do represent death, but they usually represent the fleetingness of life. Various Latin maxims and aphorisms: Memento Mori (Remember your mortality, Remember you must die or Remember you will die), Sic Transit Gloria Mundi (Thus passes the glory of the world), Carpe Diem (Seize the day), Ubi sunt...? (Where are...?) or Ubi nunc...? (Where now...?) - both of them are shortened variants of the Ubi sunt qui ante nos fuerunt? Where are those who were before us?) are very often included in the skull tattoo designs; their role is to remind us that Death comes to us all, and for this reason is important to enjoy every moment of life.
Seize the day or die regretting - a tattoo design which contains flowers (which symbolize brevity and the ephemeral nature of life) and a skull
As a symbol of the certainty of death, skulls are one of the most common symbols used in the Vanitas art
Despite the fact that the skull is often a symbol of mortality, a design which depicts a skull backed by a pair of wings (usually a pair of angelic wings, but this is not mandatory) is rather a positive design with various meanings: resurrection; death is the road to a new form of existence; the death frees one of the burdens of life; the progression of the human soul through the afterlife. Anyway, the winged skull is not always a positive design, having also many negative connotations: danger, death (death which can reach everyone everywhere), lawlessness (very common in the bikers-gang tattoos)
The skull is also a symbol of the transitory nature of life and death, and for this reason some designers choose to include a skull into more complex designs which represent the infinite cycle of the life and death.