Using a boom is, in most cases, the best way to record dialogue. Boom is a generic term for any long pole with a microphone attached to the end of it that is used to record dialogue.
The boom is used to position the microphone close to the scene to record dialogue between several actors simultaneously. The mount allows the boom operator to manipulate the microphone from one actor to another during the scene, depending on who is delivering lines. Because it is a mobile unit, the boom operator can follow moving action at a safe distance from the camera and still be close enough to pick up a clean signal from the actors.
The boom is usually held still and secure above the frame line and has a directional microphone (shotgun or hypercardoid) pointed down at the actors. (Microphones can be angled up toward the actor's mouth as well.)
During exterior shoots, a blimp-type windscreen is required to reduce the wind sounds the microphone picks up. Even when shooting interiors, always use a slip-on foam windscreen because some microphones (condenser-shotgun) are sensitive to even the most minute air movements.
The boom operator should use a set of headphones to monitor what is being recorded. The production sound mixer can give direction and speak to the boom operator through the headphones.