A stroller is ideal for carrying older babies. It is mostly lightweight and collapsible. It comes in 3- or 4-wheel designs.
A travel system on the other hand is a stand-alone stroller and a compatible infant car seat that clips into the stroller when you are walking, or clips into the car when you are driving with your baby.
While a stroller is standalone, and ideal for babies old enough to hold their heads straight without support, an infant car seat allows a stroller to be used for babies from birth, where they can lie flat on the car seat clipped to the stroller.