I'm naturally thin and it runs in my family. My eating habits will probably catch up to me someday like they have my dad, uncles, and grandpa. I still have a good 20 years though before I have to worry.
In all honesty, I eat a lot of crap. I usually skip breakfast and substitute it with coffee and a piece of fruit, be it an apple or a banana. Lunch is a big meal totaling somewhere around 1000-1250 calories, and generally consists of 2-3 sandwiches (PB&J, Ham and Cheese with mayo, Grilled Cheese with 2 slices on each sandwich), a couple cups of yogurt, a salad with romaine greens, broccoli, carrots, cheese, and ranch dressing, and another apple or banana. In the afternoon I'll eat what ever junk foot I can find, and usually head for cookies. Add another 200-300 calories from the snack. Dinner consists of god knows what, and usually weighs in around 1250-1500 calories again. Midnight snacking can range anywhere from another apple/banana, to a box of Kraft Macaroni and Cheese with 3-4 hot dogs followed by ice-cream or cookies.
My calorie intake ranges from about 3000 calories a day to 5000+ on the days I completely pig out. My diet is high is fiber, fruits, and vegetables, but also high in dairy. I drink 4-5 glasses of milk a day and I eat way too much cheese. My sodium intake is also probably pretty damn high, but my cardiologist actually recommended adding salt to my diet.
I'm 6'3"ish and weigh between 165-175lbs depending on my eating habits and physical activity over the past few weeks. I don't work out, I don't run, but I stay relatively lean. During the semesters I walk around 3 miles a day coming to and from classes. During the summer months I do landscaping here and there, and swim (along with other water activities like skiing). I'll play a pick up game of soccer (yes, I'm an America) here or there, maybe a game of ultimate or a couple rounds of disc, but I'd hardly call my lifestyle an active one.
When I played in the band I was a lot more active. Practices 4-5x a week, 2-3 shows a week, lugging heavy equipment up and down stairs far too often, and sweating like crazy. I was in the best shape I've probably ever been and I ate like a horse all day long. I slow down in the winters and usually pack on a few extra pounds. I normally shed the weight come good whether, but not this year. I had heart problems in the spring (genetic, not lifestyle related) and I haven't been nearly as active as in years past. On top of that, I spent most of last summer laid up from knee-surgery, so I'm carrying 2 winter's worth of weight right now. I'm flabby for me, but still pretty lean.
I'm probably an ectomorph. I have really long limbs, a small torso, and I can't build muscle mass to save my life. I think one thing that helps me keep weight off is that I limit my consumption of "empty" calories. I almost never drink pop/soda/what-ever.