Microphone placement   

close micing
(a few cm to .5 m) gives the best ratio of desired sound to undesired sound (ambience, leakage, noise), results in "tight" sound and "presence" too close emphasizes a particular part of the instrument/sound source and may result in imbalance or unnatural instrument sound, although this can be used to some creative advantage as well. 
distant micing
(.5 m or more) Placing the microphone too far from the sound source picks up room reflections giving a distant sound, although a mix including close mics and such an "ambience" mic can add a sense of "air" and spaciousness to the resulting recording. 

Phase cancellation effects can mar a recording using far miking due to early reflections off the floor between the sound source and microphone which combine to create dips in the frequency response of the recorded signal. Use of boundary microphones can work around this problem. 

accent micing 
is a compromise between close and distant micing which can be effective in bringing clarity and presence to a selected part of an ensemble or complex sound source. Care must be taken to be sure that the addition of an accent mic does not create imbalance in the balance and colouration between the part accented and the rest of the ensemble. 
ambient micing 
When the microphone is placed further away than the critical distance from the sound source the reflected sound is greater in intensity than the source and can be effective when mixed with close or distant miked sound to add air and spaciousness or a sense of the natural reverberation of the space the sound occurs in. 
See Also:
microphone selection
microphones (MR)
microphone accessories
mic frequency characteristics
polar response
wireless microphones