Cardioid is a microphone polar pattern. The polar pattern is how "wide" the audio "lens" is. Keep in mind, however you have to think in terms of three dimensional spheres.
You'll note that there are three kinds of cardioids, and shotgun mics are, in essence, a hypercardioid capsule in an interference tube casing that further restricts the polar pattern. Just for fun, cardioid is from the Greek "cardio" meaning heart; note the heart shape of the cardioid polar pattern.
What does that tail on the hyper cardioid add?
the reason im asking is because alcove recommended to get a cardioid in addition to a shotgun mic kit. Now im just wondering when should a cardioid mic be used. Do you ever use a cardioid and a shotgun together for a scene?
Despite GPs expertise in this area, a hypercardioid is the preferred mic for indoor usage for indie filmmakers; it mitigates (but not eliminates) the roomy echoey sound so prevalent in indie films. The problem is with the way a shotgun picks up sounds, and the way sound reflects off of surfaces in a room. 99% of indie filmmakers do not have the skill to use a shotgun properly when shooting indoors, so a hypercardioid mic will give them somewhat better sound.
If you want some idea of how difficult it is to boom properly, stick a camera on the end of a boom-pole and then keep it consistently focused on the mouths of the talent during a scene.