Cars rarely smell delightful. They're burning gasoline or diesel fuel, there's hot metal under the hood, and your brother ate a burrito just before he got in the passenger seat. There's not much you can do about your brother besides open all the windows and monitor his lunch intake, but any other automotive smells that make it into the passenger compartment spell trouble.
The scent may signal oil or coolant leaking from their normally closed-loop systems, or it may indicate dangerous exhaust gases invading your car's interior. Car and truck exhaust contains toxic gases such as carbon monoxide, so if the inside of your car smells as if you were standing behind your car, get out and get it fixed. Fight the urge to take a little nap first. So sleepy. That's the carbon monoxide talking, friends. At least open a window on your way to the shop.
The smell of burning rubber could mean you just did an awesome smoky burnout, or it could be telling you that drive belts or accessory belts beneath the hood are damaged, loose, or worn out. It could also mean that a rubber hose carrying important fluids is touching something that it shouldn't -- something that's too hot and is melting the rubber.
In any case, it's a good idea to act sooner rather than later, as these nasty smells could be linked to a much more crucial and expensive component.