Poor posture is a headache -- literally. And a shoulder ache. And a neck ache. And we can't overlook the back problems associated with a sedentary lifestyle. You might be sitting too much, and during all the time spent seated, you might be sitting the wrong way. Sitting improperly not only causes tight muscles and muscle pain, it also increases your risk of developing an inflexible spine and herniated lumbar disks, as well as conditions such as kyphosis, which is a rounded or hunched upper back, and lordosis, which is a swayback.
So if slouching and slumping don't meet the gold standard of sitting, what does? Good posture means your body is well-aligned, and well-aligned means your weight is evenly distributed, your shoulders are aligned over your hips, hips over knees -- you get the idea. It's pretty much the opposite of slumping over your desk.