Bass guitars have pickups (a type of inbuilt microphone) and need to be plugged into an amplifier (Amp) to be heard. There are two basic types of pickups, one being passive (found in traditional bass guitars such as the Fender Precision and the Fender Jazz) and the other being active (found in many more recent basses such as Music Man and Alembic).
Active pickups are more powerful and require a battery which fits inside the body of the bass. Some players prefer the bright sound of active pickups for slap playing, while others prefer the traditional sound of passive pickups. Before buying a bass, try out both types in a music store and if possible, get a professional player to help demonstrate the sounds they make before choosing a particular instrument.