There are a couple of different options in terms of microphone type. ‘Shotgun’ mics connect directly to the camera via a cable, and are usually affixed to the top of the camera via hotshoe. Often called ‘directional’ mics, they mitigate ambient noise, like wind or traffic, allowing the subject to be the sole focus of the recording. ‘Lavalier’, or ‘lapel mics’, are the smaller devices you often see attached to an interviewee’s clothing on TV. These are much more discreet, and are pointed directly towards the speaker’s mouth to reduce the amount of ambient noise captured by the mic.
Dedicated camera microphones also offer more control over the audio recording than your camera’s internal microphone. While some higher-end cameras will give you rudimentary control over volume levels, a dedicated microphone will allow you to control volume, sensitivity (i.e. how much audio you want the mic to collect) and also direction. This last point is key. Imagine you’re recording outdoors, and there’s wind and other ambient noise to contend with. A directional mic, with a windshield, will allow you to focus the mic directly at the source you’re trying to capture - e.g. an interviewee - and be confident you’ll record just that sound with vastly reduced wind sound. Likewise, an omni-directional camera mic captures sound from every angle making it the ideal choice for interview or discussion videos recorded in a quieter, self-contained space.
Many mics also give you control over things like frequency response or noise reduction, which allows you to cut out certain frequencies at source, reducing the amount of post-production you’ll need to do on a waveform. This could be low frequencies, like the rumble of traffic outside the building, or it could be high frequencies like that creaky door down the corridor.