Building the thighs is focused on helping you develop a winning physique for a bodybuilding challenge. It is also for people building their dream body that want the most aesthetic appearance possible. Great size on the legs, comes via lifting astounding, heavy reps with core exercises such as squats and leg presses. You're legs will have a hard time growing if you don't do some really heavy weights with low reps per set. Some people with genetically gifted legs may get away with doing less than heavy reps.
To achieve championship level thighs you'll need shape, mass and separation between each of the quadricep muscles know as the rectus femoris, vastus medialis, vastus lateralis and vastus intermedius. You must also keep in mind that your lower body needs to be in proportion to your upper body. You don't want to under-do it or over-do it with your legs. Don't forget your calves either. Many people develop their thighs but neglect their calves due to time restraints or lack of interest.
When building the thighs there are three priorities to keep in mind.
Building the quads is done with a mass-building exercise known as Squats. They also build the gluteals and Squats are well regarded as an essential exercise for the development of your thighs.
When starting out with squats, as you go down in movement the thigh bears most of the stress, the farther you go down the more the burden is transferred to the hamstrings. At the very bottom of the movement, the buttocks takes the strain. Another handy exercise that is sometimes necessary to target the quadriceps and lessen the impact on the lower back are Front Squats.
What can't be overlooked in your effort to be a bodybuilder is a solid diet. Muscle separation and definition cannot exist without the use of a good diet. To learn about a variety of diets see this page, there are many. If one doesn't work for you, try another diet until you succeed. Personally, what works for me is the HCG diet. I've tried other diets before finding the one that worked for me.
Besides dieting when building the thighs, you need a variety of exercise that target your thighs such as Leg Extensions and Leg Curls. Hack Squats can even be added at some point and they add hardness and definition. Squats and Lunges indirectly work the hamstrings as well.