A cardioid (from the Greek καρδία "heart") is a plane curve traced by a point on the perimeter of a circle that is rolling around a fixed circle of the same radius. It is therefore a type of limaçon and can also be defined as an epicycloid having a single cusp. It is also a type of sinusoidal spiral, and an inverse curve of the parabola with the focus as the center of inversion.
The name was coined by de Castillon in 1741 but had been the subject of study decades beforehand. Named for its heart-like form, it is shaped more like the outline of the cross section of a round apple without the stalk.
A cardioid microphone exhibits an acoustic pickup pattern that, when graphed in two dimensions, resembles a cardioid, (any 2d plane containing the 3d straight line of the microphone body.) In three dimensions, the cardioid is shaped like an apple centred around the microphone which is the "stalk" of the apple.
A cardioid pickup pattern means pickup to the front of the microphone, and to a lesser extent the sides, with good rejection of sound to the back of the microphone.
Cardioid microphones are recommended for vocal applications, live taping/recording, and most other situations where the acoustics of the recording environment are good, but not perfect.
When indoors, and recording vocals, something handy is a pop filter. Pop filters are a light, nearly transparent mesh placed over a wire or plastic frame, and held in place over a microphone diaphragm with a special clamp. This is useful in reducing "plosives", or exaggerated P and S noises.
One thing to avoid are pop filters with spring-loaded clasps. Only pop filters that attach to your microphone stand by clamp are helpful.
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