Athleticism refers not only to physical development but to motor skills. The latter applies to your ability to execute different movements, techniques and coordination that are involved in a particular sport. In general, a person who has a high level of athleticism is one who is capable of executing all or most of the basic movements that are seen in many sports.
Most often this includes agility movements such as cutting actions, zigzag running, skills such as jumping, throwing, kicking, hitting, fielding, and abilities such as exhibiting quick bursts of energy, short acceleration runs and so on. Being able to execute these athletic movements individually or in combination is related to a great extent on your levels of strength, flexibility, endurance, coordination and early development.
For example, I recall an international volleyball match, which included a team from the Ukraine. They had a tall junior player who was their best spiker, blocker, setter and receiver. He was as good as, if not better than, the other players who specialized in one skill (as for example a setter) and were good in the other game tasks. He was a classic example of high-level athleticism.
One of the best ways to develop athleticism is to be involved in many sports as a youngster. In so doing, you develop the coordination needed to execute the basic skills and actions that are seen in most sports. For example, once you learn how to run well in one sport, it will carry over to the other sports. Thus, the more varied your background, the greater will be your ability to execute just about any skill and thus play the sport successfully.
This is one of the reasons why I strongly advise against specializing in only one sport at a very early age. By playing only one sport, after you learn the rudiments of the basic skills for that sport, you only learn more strategy. You do not develop the ability to execute new and different movements or other skills that are needed in other sports or that may be needed when playing on the highest levels.
You may say that this is immaterial if you only play one sport but it is not. As sport and sport technique continually evolves you must prepare the youngster with the ability to master any new skills that may be required on the highest levels of play as an adult. This is one reason why so many youngsters who specialize and excel when they are very young often do not do as well during or after the teenage years.
Athleticism is closely related to physical abilities. The greater the development of your strength, flexibility, speed, quickness, etc., the more able and the better equipped you are to execute the different skills. In essence, your skills can be improved not only by improving your technique, but by developing the physical abilities as they relate specifically to the skill in question. This means that your physical abilities should be coupled together with technique. In this way, your technique and physical abilities will increase at the same time allowing you to display the best execution and overall game play.
For more information see Build a Better Athlete. It is the only book that shows execution of the basic skills with sequence pictures taken from live digital film. In addition it explains the need for the major physical qualities and how they can be developed. For related information on specific sports see Explosive Running, Explosive Golf and Secrets of Russian Sports Fitness and Training.