The words 'doorbell' and 'chime' are often used interchangeably, but not knowing the difference between the two could result in buying something that you already own… and leave out the missing piece that you actually need.
The doorbell refers to the entire mechanism, and consists of a push-button that can be pressed by visitors outside your house, a chime and, sometimes, the wiring that connects them;
The chime is the part of the doorbell that is located inside your house, making a sound whenever the push-button is pressed:
it can either be permanently wired and attached to your wall or be portable;
while most houses have one chime, it's possible to connect more than one to the same doorbell system.