Probably one of the most overlooked aspects of a video production is sound. Picking the right microphone for the job can be a pain, mainly because different microphones pick up sound in different ways and that can cause a headache when you're playing it back later. So, it would behoove you to understand the difference between an omni-directional and a cardioid, because not all mics are created equal.
Omni-Directional Versus Cardioid: Breaking It Down
"Omni-Drectional" and "Cardioid" refer to the pick-up pattern of a microphone.
"Omni" means "all," so no surprise, omni-directional means, "all directions." It has a pick-up pattern shaped like a large sphere and will pick up sound from any direction.
For lavalier mics ("lapel" mics), you pretty much always want to go with omni-directional, because you never know which way you're going to need to attach the mic to the onscreen talent. The mic's location will be dictated by the talent's wardrobe. These mics are ideal because they will pick up the talent's dialogue, and just a little of the atmosphere. And because they are attached to wardrobe, they can move slightly out of place and it's good to be able to pick up the sound no matter what.
"Cardioid" is partly derived from the Greek word "kardia," or "heart." Thus it has a pick-up pattern in the shape of a heart and only picks up the sound directly in front of it. It mostly ignores the sound coming from the sides or from the back.
These are most effective with shotgun mics. They will pick up the sound in front and will virtually ignore all the sound from other sources. The only problem is if the mic gets moved out of place during takes and the sound starts to become weak.
The difference between a "lav" and a "shotgun" is detailed in this episode of Reel Rebel, and shotgun mics in general are covered here. There's even more microphone fun in this episode where the Zoom H4 is reviewed.