A dialer is software that places outbound phone calls automatically from a contact list, so sales agents don't have to look up and dial each number by hand. At minimum, a dialer reads the list, places the call, routes the answer to an agent and logs the outcome. In practice, a modern dialer is the operating system of a calling floor — the layer that decides how many calls to dial, which numbers to dial from, what to discard before the agent hears it, and how every outcome feeds back into the next call.
"Dialer" is an umbrella term. Inside it sit four common architectures: preview (one call at a time, agent reviews before dialing), progressive / power (one call per agent, dialed automatically when the agent finishes), auto dialer (loose marketing term, usually a progressive variant with light filtering) and predictive (many parallel calls per agent, paced by live campaign data). The difference between them is measured in live-talk minutes per agent per hour — and in closed deals per shift.
What separates a good dialer from a bad one is rarely the dialing keystroke. The bottleneck is what happens around the dial: how voicemail is filtered, how caller-ID reputation is preserved against Truecaller and on-device spam screens, how idle time hides between calls, and how the system feeds outcomes back to the pacing model. Most dialers automate the easy part and leave the rest to the agent.