Body typing, or somatyping as it is formally called, actually comes from psychology. A psychologists by the name of William Sheldon attempted to assess people's "temperments" based on the structure of their bodies back in the 1940's. The idea has been largely rejected since then, though the three classifications of ectomorph, mesomorph, and endomorph are still used. In my view they are a valid way to describe a person's body shape and to a limited extent their propensity for muscular development. A person's somatype can be altered with changes in age and body composition, namely fat to muscle ratio. The wikipedia article on the topic states that a mesomorph may resemble an endomorph as he/she ages with a decline in muscle and increase in fat. Likewise an endomorph may resemble an ecto or mesomorph if he loses large amounts of body fat through starvation or dieting and exercise.
It is my contention that muscle and bodyfat levels do not a somatype make. Examine the skeletal structure of the person observed. A mesomorph will have narrow hip bones and wide shoulders due to collarbone and ribcage size. An endomorph will have wide hip bones and shoulders of the same width. An ectomorph will have average to narrow hip bones with similar shoulder width though most notable ectomorphs have small bones, especially in the arms. Classically speaking, the endo has the largest bone size, the ecto the smallest, and the meso somewhere in between. A classic mesomorph trait is a torso to leg length ratio of 1. That is, if your upper body is as long as your lower body, or if it looks like you have short legs when you stand up straight, you have a meso trait.
Ability to develop muscle and strength is somewhat related, though not completely. In my own experience I knew a guy in high school who was short and slender, but remarkably strong. His bone size said ectomorph, but his shoulder to waist ratio was high. He had a muscular physique though he didn't work out. He did gymnastics as a kid. Interpret that any number of ways. The super heavy weight Oly lifters are usually endomorphic. The large bone structure and especially the large hips provides the ability to handle heavy loads. Olympic male sprinters are usually ecto/meso hybrids. They are tall and slender making them aerodynamic. Most notably they have very narrow hips. But they are also well muscled. Female Olympic sprinters have no hip to waist ratio to speak of. This trait makes them more aerodynamic, and as you can see it makes a difference.
There are many factors that dictate size and strength. Bone structure is just one of them. Other factors include muscle fiber density, muscle attachment lengths, fiber composition, natural androgen levels, etc.
Somatypes are a point of reference.
Some individuals will fit neatly into one category. Many individuals do not and are somewhere between two of the classical types. To get an idea look at your skeleton first. Look at your bone size, your waist to hip ratio, and your torso versus leg length. Consider your natural muscle levels and propensity to gain body fat second.
Before I moved away from Charleston I hung out with an ex-marine. He was built like a tank. He had 18 inch arms easy, even without lifting, very thick shoulders, and his torso actually looked longer than his legs. I asked him once if he'd ever lifted weights, because being a weightlifter myself I recognized he had the talent for it. I'll never forget his response:
"Dude, when I was in the marine core I worked out once for like 6 months. All of my friends said I made them sick 'cause they had been at it for years and still didn't look like I did. I quit after a while. I wasn't really into it."
I about died.
To answer your question DWhite, I am a classic ectomorph of about 6 foot 5. If I straighten my posture some more I will get to be about 6'6. My ability to gain muscle looks to be about average, though my slender musculature puts my potential relatively low. The upside is that dieting is not necessary for me. Definition is forthcoming. The muscle I do gain looks more than it is because of my bone size. And my height forces people's eyes to move around in order to scope me out, adding to the illusion of size.
I am working on a T-shirt concept:
"I am not a bean pole. Are you a puff ball?"