There are a lot of different types of thin guys.
We have the:
A. Untoned semi-thin: they don't know what a dumbbell is and whine non-stop about how "working out is too hard *insert whiny noise here*" Body is kinda flabby, with hints of fat, and zero definition of any sort.
B. Skin-tight super-thin: Skin tight cause they eat practically nothing and run cardio all day. You can see practically every muscle in the body, but there is so little muscle it's like viewing a walking skeleton. The slightest touch looks like you might break them.
C. Athletic toned and thin: Good proportion of muscle on the body. Not a thick build. But nevertheless succeeded in making "thin" work, and work well I might add. You can see good muscle definition, and have made a successful balance of cardio and resistance training to attain this attractive look.