Time

The Merriam-Webster Dictionary defines time thusly:
 * 1a: the measured or measurable period during which an action, process, or condition exists or continues b: a nonspatial continuum that is measured in terms of events which succeed one another from past through present to future

Time is percieved as a continuum by some, and by others a constant, linear progression. Still others consider time to be more of a fluid, with currents and eddies that change as events are altered or changed. Reguardless of one's definition, 'time' is almost always considered to be the 'fourth dimension' of space (the first three being physical location in space), signifying a change in location as time itself progresses.
 * It can be speculated that this is sense that is reflected in the name TARDIS, or Time And Relative Dimensions In Space. 

One of the constants in most of the 'lesser races' of the galaxy has been that time, once passed, cannot be re-explored or revisited. This is not the case for the ancient race of Time Lords, who discovered fairly early in their development how to travel back and forth through Time. The great Time Lord Rassilon, in conjunction with Omega, discovered a way to pass back and forth through time by entering a structure known as the Time Vortex, a sort of pipeline that came into contact with every possible moment in time. The devices Rassilon invented to do this were known as TARDISes.

Though the advent of the TARDIS made time travel possible, it certainly did not make it easy or painless-- so much energy is required to make the journey that every TARDIS is powered by a black hole, known as the Eye of Harmony. If contact with this black hole is severed for any reason, a TARDIS can be considered 'dead'-- though it is possible to use emergency power to attempt another jump, doing so is risky at best.
 * (DW: Rise of the Cybermen, The Age of Steel)

The Time Vortex is the primary means of transit through time, though the actual nature of time itself is still very much up to debate. Two of the races that have mastered travel through time, the Daleks and the Time Lords, seem to have taken any information of its nature to the grave with them, as both races are now virtually extinct.