What exactly would happen if an asteroid/meteor/comet of substantial size were to hit the planet?

I know that's a tough question to ask, because we've never experienced one directly, but movies have always painted my impressions oddly. Armageddon, which is about as credible as a my four month old godson on this topic, gave me the whole "wave of fire across the planet" idea, Walking With Dinosaurs told me about the blast wave, and a bunch of other varied media and common sense gave me knowledge of the post-impact fallout.

But the thing is, I have no idea what a factual representation of what we think an asteroid impact would look like would be. Let's say a projectile of sufficient mass to do some actual damage, were to hit the Earth. Maybe as large as the one that killed the dinosaurs, or a bit smaller. Nothing too catastrophically huge, and it hits a solid landmass, so no titanic tsunamis.

What exactly would go on, and how could people survive that? Every time I talk about this with my friends, or I see this debate go on, everybody comes to the conclusion that, as a species, we'd die. Why is that? I know the ejecta would seriously screw us, and that'd likely be the end of the survivors, but what would the main impact do? Waves of fire across the planet, massive global earthquakes, or just regional damage up to a point? I tried researching this, but all I could find were the usual "doom and gloom, almost guaranteed death for the species" explanations, but nothing concrete about what actually would go on.