In a car we equate the fuel it will take for a trip in terms of distance. In rockets we equate the fuel required for a trip with the delta V. Delta V stands for change in velocity. The variable is written <math>\Delta</math>v in mathematical formulae, but this important concept is used so much in rocketry that it has become known just as well by its phoenetic spelling. A typical Hohmann trajectory requires two burns, an injection burn and a circularization burn. The total delta V is the sum of the changes of velocity of each burn.
One may wonder why each delta V is added to produce the total delta V when energy is proportional to the square of the velocity. The reason is that the when the rocket is at a higher velocity the burn occurs over a greater distance. Since work is equal to force multiplied by distance more work is done on the rocket. However, since energy was wasted initially to get that fuel up to speed the rocket falls far short of cheating the laws of conservation of energy.
The relationship between the amount of fuel need to go a given delta V for a rocket of a given size is known as the rocket equation.
delta-V calculator (Requires Java-capable browser.)