Many people seem to think that lifting weights is lifting weights is lifting weights. That's not the case. There are lots of different weightlifting protocols -- the 5x5, the "gym rat" 3x10, HIT, etc. Each protocol produces a different effect in the trainee, and each protocol can be sorted into 1 of 2 general categories: Strength Training and Bodybuilding.
To understand these categories, we must understand the goals of each. In Strength Training, the goal is to get stronger. To put it more precisely, the goal is to increase the amount of weight that you can lift for a single repetition. This is achieved by increasing the power production capacity of muscle fibers and by improving the ability of the nervous system to recruit those fibers.
In Bodybuilding, the goal is to increase the visible amount of muscle on your body. This is achieved via hypertrophy (the process by which the body creates new muscle cells and/or increases the size of existing ones) and by reducing body fat.
There is some overlap in these two goals. In the process of building strength, you will begin to appear more muscular. In the process of building muscle, you will start to get stronger. But the two are not the same.
There are a number of easily noticeable differences between Strength Training and Bodybuilding programs:
To build strength, you need to lift heavy weights, weights that you won't be able to lift for a whole lot of consecutive repetitions. This means that most Strength Training programs will use sets of no more than 8 repetitions. Most commonly, you'll see sets of 5 or 3, and more advanced Strength Training Programs often call for heavy singles (sets of 1 rep).
For hypertrophy, you want to cause as much muscle fatigue as possible. This means more reps with lower weights. For this reason, most bodybuilding programs will use sets of at least 8 reps, often going as high as 20 or "to failure," which means as many reps as you can possibly do with the weight.
Strength is most efficiently built through movements that mimic real-world actions: picking things up, pushing things around, etc. This means that Strength Training programs rely heavily on compound movements: complex movements involving more than one joint. Examples would be the Squat, Deadlift, Bench Press, Overhead Press, Power Clean, Bent Over Row, Pullup, etc.
In competition, bodybuilders are judged on a set of criteria that include muscle definition, proportion, etc. For this reason, bodybuilders often try to isolate specific muscles in order to precisely manipulate their physique. Isolation and machine exercises such as tricep pull-downs, preacher curls, pectoral flys, leg extensions and calf raises are often found in a Bodybuilding program.
Your body works as a unit. When you pick up a heavy box, you're using almost every muscle in your body. Therefore, Strength Training programs usually train the whole body in one workout, though some Strength Training programs use a split in which the upper body is worked one day and the lower body is worked the next. Your body needs time to recover from these kind of large scale workouts, so Strength Training programs usually involve 3-4 workouts per week.
Because they want to fatigue a specific muscle or muscle group as much as possible in order to induce maximum hypertrophy, bodybuilders often only work out one specific area of the body (chest, back, shoulders, triceps, biceps, abs, legs, etc.) per day. Therefore, Bodybuilding programs are often designed so that the trainee works out 5-7 days a week, working a different area of the body every day. Bodybuilding programs don't have as many rest days built in because each workout stresses only a small area of the body, creating less systematic fatigue and necessitating less recovery time.
Not all programs are created equal. It's important to define your goals and pick a program that matches those goals. If your main goal is to pump up your "beach muscles," pick a Bodybuilding program. If you're training for a sport or you just want to be the strongest guy you know, follow a Strength Training program.