dominar, a cardioid microphone discriminates somewhat in its pickup of sound, since in the back of the microphone it has almost no response at all (in fancier terms it has a "null" in its response at 180 degrees). Any sound that arrives from the front or sides of the microphone will be picked up, though.
An omnidirectional microphone is similar except that it also picks up sound from the back of the microphone, in addition to the front and sides. Cardioid and omnidirectional aren't opposites, in other words; "directional" and "non-directional" would be opposites, but those terms aren't quite the same as what you asked about.
The directionality of a cardioid isn't nearly as sharp as many people seem to imagine. It isn't anything like a flashlight beam, for example; no conventional microphone has that narrow a pickup pattern. Even sound that arrives from 90 degrees off axis (all the way to one side or other, and/or above or beneath the mike if it is horizontal) is reduced only barely enough to notice. When people here talk about the use of cardioids to reduce audience noise, it is because they are recording clear enough stereo that the listener's brain can filter out some of the unwanted sound in playback. Beyond enabling stereo pickup to occur, though, cardioid microphones themselves don't really do much "filtering out" like that. Cardioid is still a big, round 3-D blob of a pattern.
In practice there are some things that go along with a microphone's being directional or not. Directional microphones are sensitive to wind, breath noise (popping on consonants with closely miked singing or speaking voices, for example) and handling noise or physical vibrations; omnidirectional microphones, if they are of a type known as "pressure transducers," are nearly immune to these problems. And directional microphones emphasize the bass and lower midrange of most sound sources that are in close proximity to them while pressure transducers don't have any such "proximity effect."
Just speaking of condenser microphones, since they're the type most commonly used for high-quality recording and broadcasting, directional microphones have a low-frequency rolloff that generally starts somewhere between about 50 and 100 Hz, while pressure transducers (remember, all pressure transducers are omnidirectional but not all omnidirectional microphones are pressure transducers) can have full sensitivity down to the lower limits of human hearing and even beyond.
Finally, for reasons of physics, "omnidirectional" microphones of normal size aren't omnidirectional at high frequencies, so you still have to take care which way you aim them--strange though that may seem at first.
--best regards