Omnidirectional models pick up sounds equally from all directions and are frequently used for on-camera broadcast interviews, where two persons are being covered by a single handheld mic. Speaking of interviews, another handy mic in your kit may be a lavalier type, which essentially puts a miniature mic capsule onto a small clip-on mount. This is ideal for capturing dialogue in a noisy setting or for interfacing with a pocket-sized, wireless body-pack transmitter.
The most directional mics are shotgun models. They're ideal for general video work, whether camera-mounted or used with a boom pole. This design places a small condenser transducer within a long tube, resulting in an extremely directional pickup pattern. Typically, the length of the tube determines the shotgun's reach, and available models range from about six inches to two feet in length.
Many microphones designed for the DSLR and camcorder market include an accessory shoe-mount for placing the unit onto your rig. Actually, placement atop your camera isn't ideal. A boom mic or close-in handheld unit is preferred because bringing your mic closer to the sound source offers better reproduction, with less environmental sounds and noises and more dialogue. That said, sometimes the environment is exactly what you're trying to capture, say, if you're shooting a Mardi Gras parade or rain-forest ambience, where a wide sound field is just the ticket. The other side is a dose of reality—if you're working alone, a camera-mounted mic is a necessary compromise and a huge improvement over the camera's built-in mic.
The key thing to look for in an on-camera mic is a mount that incorporates some kind of audio shock isolation from handling noise and mechanical clatter, such as autofocus servos. For outdoor use, another must-have is some kind of foam wind filter to protect the mic from being overloaded by blasts of air. For more extreme conditions, a fur-style muff, such as the RODE Deadcat or Que Audio's Wombat, fits over your mic to provide additional wind protection.
Some models that incorporate shoe-mounts are the Nikon ME-1, RØDE VideoMic/VideoMic Pro and Sennheiser MKE 400. Among those models that include a detachable shoe-mount are the Audio-Technica Pro 24-CM, Azden SGM-1X, Sony ECM-CG1 and Que Audio Mini Shotgun. Any of the latter can easily double as a close-in mic by adding a simple 1⁄8-inch stereo extension cable, found at any Radio Shack®, but keep your overall cable length to a maximum of about 15 feet to avoid picking up interference.