An example of training a movement is to perform a barbell squat. The movement is the squat, the body parts required to perform the squat are the quads, hamstrings, glutes, core, calves and the spinal erectors. This is a compound movement (multi joint) and is a very efficient exercise that uses a lot of muscle groups.
An example of training a body part would be using a leg press machine. This is specifically targetting the quads (as well as the glutes to an extent depending on the depth) but doesn’t use any of the stabiliser muscles like the core and back.
Both of these methods have their place and it depends on the person’s goals and there is nothing wrong with using both! Many people seem to get in the mindset of black or white. There is nothing wrong with some shades of grey!
I will typically recommend beginners to start with a full body workout that is comprised of many compound exercises. The reason for this is that this is the quickest way to build a base of strength. Once a strength base is achieved, then the specific goals of the person will be used to design a program that meets their needs. Are they a triathlete, bodybuilder, powerlifter or just wanting to look good naked?
One of the best examples of training the movements are some of the Russian powerlifting programs like sheiko and smolov where the competition lifts are basically all that is done in each training session. The methodology is that the more you practice the specific movement, the better you will become at it. Whereas, the Westside sytem (a popular American program) utilises many different sorts of accomdated resistance like bands and chains. The reasoning for this is that they believe that if you are placed in a more difficult sitation than in the competition lift you will be able to power through it e.g. box squatting with chains. This just shows that there is more than one way to skin a cat and is one of the reasons why experimentation is key. One thing is for certain though, progessive over load is a requirement for strength and hypertrophy (or maintenance of LBM) and your program should include this. As they say, what doesn’t challenge you doesn’t change you.
An example of training a body part would be a typical bodybuilder split program that is based around training a certain body part on a certain day. An example of this would be a 4 day split like the following:
Monday: back and biceps
Tuesday: chest and triceps
Thursday: quads and hamstrings
Friday: shoulders,calves and abs
These type of splits are designed to target a specific muscle on the day. Depending on the level of the trainee, the sets and reps can vary greatly (especially in a poorly designed program). Although this type of training is focussing on a specific muscle group, many of the same compound exercises can be used e.g. squats, deadlift and bench press. These are typically followed up with isolation (single joint) exercises for the target muscles.
In summary, there are MANY different types of training methods to choose from and it is important to set your short and long term goals to help you determine which type of program is best suited to you. The worst thing you can do is program hop. If you decide on something, stick with it for a decent period of time and if it ain’t broke don’t fix it.