Booms are a commonly used piece of sound equipment that allow you to position the mic close to your subject while staying just out of shot. Though a number of factors may influence the limitations on mic placement potential (for example, the chosen camera frame and the overall design of the set), the overall preferred method is to boom from above, or below if absolutely necessary. You should never record from the sides.

Holding the boom pole above shot ensures that the optimum pickup area of the mic is directed towards the subject's mouth, and then towards the ground – which we assume won't be generating much noise. This will give you natural-sounding dialogue, which is well-isolated from the ambience of the surroundings. In addition, the boom and mic will both be well out of the actor's way, leaving them free to roam the scene and not feel distracted by the sight of it.