Because the english wikipedia is pested by bad references (mostly third class fitness sites that oversimplify the whole thing) I will cite from the German Wikipedia (with translation by me).
Diese Idee ist heute medizinisch widerlegt. Das Konzept taucht heute fast ausschließlich im Fitness-Bereich auf, um ein dem Körpertyp angepasstes Trainingsprogramm zu erstellen. Die Begriffe und umsomehr die dahinterstehende Theorie sind aus heutiger Sicht mehr als fragwürdig; wissenschaftlich relevant sind sie nicht.
This idea is medically refuted nowadays. The concept only comes up with fitness relation to create a training program customized to the body type. The term and even more the theory behind it are more than questionable from a present-day perspective; they are not scientifically relevant.
In der Humanbiologie allerdings werden die Körperbautypen zur Beschreibung des individuellen morphologisch-anatomischen Aufbaus eines Menschen genutzt. Die Bestimmung des Typs erfolgt dabei über die Messung der Breite der großen Gelenke (z.B. Knie) und Einsetzung in eine Formel.
In human biology, however, these body types are used to describe the individual morpologic-anatomic structure of an individual. The typing is done by measuring the size of the big joints (eg knee) and inserting them into a formula.
So, as Moses commented, these types are still in use.
Die Einschätzung nach Körperbautyp wird insbesondere in der Sportmedizin angewandt, um ein angepasstes Trainingsprogramm zu erstellen.
These classifications by body type are used especially in sports medicine to create a personal training plan.
But however:
Die heute gebräuchlichste Form der Somatotypbestimmung geht auf Heart und Carter A Modified Somatype Method von 1967 zurück [...]
The method used today dates back to Heart and Carters A Modified Somatype Method from 1967 [...] [link included by me]
Although I had to completely rewrite my answer, my main point is still valid. But I have to modify it a bit:
What most of the fitness sites tell you is still crap. They give you a checklist and you have to check for certain features, and then they put you inside one class and make you believe that you have all of those features that are tied with it, this a bad oversimplification in my eyes.
Which features are measured today?
Körperhöhe, Körpergewicht, Hautdickenmessung an vier Punkten, zwei Umfangsmessungen an Extremitäten und zwei Knochenbreitenbestimmungen.
Height, weight, skin thickness at four points, two circumference measurements on limbs, two bone width measurements.
And the most significant part is that there are not only the three extreme types but far more:
Die meisten Menschen [weisen] Merkmale aller drei Typen auf. Man unterscheidet in der Sheldonschen Typologie etwa achtzig Untergruppen. Die Fettanspeicherung, der Muskelaufbau und der Skelettbau sind dabei eng korreliert.
Most people show signs of all three types. Sheldons typologie know around eighty subgroups. Fat-storing, muscle development and skeletal structure correlate closely.
So what can you learn from this?
You are probably not an extreme type, but even if you are a complete ectomorph by the definition of sports medicine, you wont gather muscle mass as fast as other people but you will excel in endurance activity.