A zoom moves internal groups to change focal length continuously — 24-70mm, 70-200mm, 18-135mm. One lens covers what would otherwise take three or four primes, which is why zooms dominate event, wedding, wildlife, and news work where changing glass means missing the moment.
The compromises are physics. Correcting aberrations across an entire range rather than one point means more elements, more weight, and usually a smaller maximum aperture. Many consumer zooms are also variable-aperture, quietly losing light as you extend — f/3.5 at the wide end becoming f/5.6 at the long.
Constant-aperture professional zooms close much of the quality gap and hold f/2.8 throughout, at considerable size and cost. The honest way to choose is by scenario: a zoom when you cannot control your distance from the subject, a prime when you can and want the aperture or the sharpness.