Bodybuilding and Weight Training/Bodyweight Training
A popular but less known method of bodybuilding is using solely your body weight to build muscle. This method has advantages in the fact that the exercises are more natural movements, and are less concentrated on individual muscles, which lowers the risk of muscle imbalance and injury. Many athletes use primarily body weight training.
Push-ups are a well known and effective exercises that is easy to learn and very efficient. Primarily building the pecs and triceps, push-ups also build shoulder-muscle as well as helping to develop a stronger core.
To execute a push-up, lie face-down on the ground with your legs together and your hands pressed flat-palmed against the ground just wider than shoulder-width apart, parallel to your neck. Keeping your body rigid, slowly push up with your arms until they are almost fully extended, with your feet at a ninety-degree angle with your legs, and your toes bend against the ground supporting your legs. Hold for a second at the top, then slowly lower yourself back down, making sure to keep a straight core, until your nose is just above the floor. Repeat reps as able. It is recommended to do three sets of push-ups to maximum repetition every workout. Also, for a more thorough workout change your grip on the ground to thumb and forefinger of each hand touching, just below your chest, and also hands spread wide out on either side. These three grips will allow more thorough muscle development.
If a regular push-up is too challenging, there are variants to make it easier. Firstly, use your knees as a pivot instead of your feet, just rest your knees on the ground instead. If this is still hard to keep good form, use something such as stairs to create an incline so that its easier to complete.
Most bodybuilders do not use push-ups to develop these muscles, as they can do so many; it is more of an endurance workout than a muscle building workout. True muscle building workouts involved sets with a very maximum of 12 reps, and it's usually less than 8 reps. To make the workout more of a challenge, one can use high objects to rest ones feet on, to make the body more of a vertical angle to increase the resistance. Over time, a bodybuilder could work to a vertical position, completing handstands.
Pull-Ups, Chin-Ups 
Another excellent upper-body workout are pull-ups and chin-ups. Chin ups are when you hold onto a bar, with both palms facing you, then lowering to extension, then lifting until your chin is higher than the bar. These exercises primarily strengthen the latissimus dorsi, but help to tighten arms (biceps) as well.
To perform, it is recommended that you have a round bar at just over head-height, possibly suspended in a door-way. These pull-up bars are cheap, easy to install, and most definitely a must have. If for some reason you do not have access to a bar, use the top of a door-frame, throwing a folded towel over the top to pad the grip for your hands. Assuming you have a bar, grab the bar with both hands, shoulder-width apart, and your palms facing either away from you or towards you (perform more reps of whichever grip you are weaker at by doing this grip first, but make sure to do both ways for good muscle balance). Then, letting all of your weight hang from your arms, pull yourself up until your chin is above the bar, then lower so that you are still hanging off of the ground. Perform reps until fatigue, and three sets of maximum repetition is also recommended per workout.
Most gyms have pull-up bars, as well as machines that work the latissimus dorsi, and many bodybuilders simply work this muscle group at the gym. When regular chin-ups and pull-ups become too easy, many add weight by simply holding dumbbells between their legs or using weight belts.
A very simple, yet powerful exercise is the squat. Many athletes add weight to this activity when the form they use isn't good and doesn't develop leg muscles properly. Most people would benefit from not using weights initially.
To complete a full squat, place your feet approximately shoulder width apart (a good method is jumping and landing, this is close enough) then lower your body slowly to the ground, while keeping your feet flat, until you're resting on your feet for balance. There are different depths to lower to, but a full range develops all the muscles equally. To increase difficulty, either add weight or attempt a one legged squat, "pistols".
Basic Muscle Groups 
The basic muscle groups that body builders normally train can be classified as upper-body muscles and lower-body muscles
Upper Body 
The important upper body muscles are Shoulders, Chest (Pectorals), Biceps, Triceps, Lattismus Dorsi and Trapezius.
Some of the common exercises used for these muscle groups are
Shoulders - Military press Chest - Bench press Latissmus Dorsi - Lat Pull Biceps - Barbell curl Triceps - T-Rod
Military press and Bench press can be done with either barbell or dumbbells
Lower Body 
The important muscle groups are Gluteals, Hamstrings, Quadriceps, Hip Flexors, Gastrocnemius, Soleus, Tibial Anterior
Some common exercise are Hamstring, Glutes, Quadriceps, Calves - Squats (will target different muscle groups depending on stance), Deadlifts.
Squats can be done with either body weight, barbell, dumbbell, kettlebells, Smiths machine and a lever system.
There are many variations of the squat such as front squats, back squats, box squats, cambered bar squats, jump squats, one legged squats (a.k.a. "pistols") etc.