Here is a handy tool to figure out your body type: http://www.bodybuilding.com/fun/becker3.htm
I'm definitely mostly an ectomorph. I have a small frame and find it difficult to put on muscle. I can gradually put on weight over time, but I can drop weight very quickly and return to a previous weight fairly shortly as well; just not exceed it.
Lately I've been playing with my body. Seeing how it reacts to no cardio and resistance training; then metabolic cardio (still with resistance training days). I've tried kickboxing, TRX, metablic pilates, resistance circuits with light weights and high reps and no rest ... all for the sake of finding what WORKS for me. I'm not very far along in playing with my training program, but the results have been minimal. That's not a discouragement, however. I know that my body-type plays a huge role.
What's interesting is growing up I always figured I'd hit late 20s and suddenly balloon. My entire family registers on the morbidly obese scale (note: I don't have a relationship with them, so no I can't help them!) and it all happened for them in their late 20s. I figured I had genetics against me. Now that I've found I'm still staying trim (clearly I do workout often!) I am beginning to accept that my family didn't have a genetic problem; they had a lifestyle problem! Lack of exercise, fast food, sugary sodas, large portions and processed, microwavable crap filled their bellies! It's NO WONDER they ended up where they did.
I was always 5-10 lbs underweight throughout my gradeschool years; and it wasn't until I hit my 20s that I developed; fat finally decided to lay down and fill me out. When I hit 130# I was done begin skinny fat and decided I'd start doing cardio. I hit 115# and decided I wasn't looking shapely and so hit the weights. Now hang out between 120 and 125# depending on my diet (I can drop 5# in 5 days). I'd love to be 130# again; except I want all that new weight to be muscle.
This is a journey of trial and error; what works for some won't work for others, and I have to be patient and very aware of the changes I make to my training and diet. Just like everyone else I'm trying to figure out what I need to get the results I want!