Line in is an audio input normally around 150mv line sensitivity and can be used for devices such as tape players,cd players,mp3 players etc.
It cannot be used as a microphone input as you would hardly hear it, a microphone input needs to see an input sensitivity of around -5 mv input,as cd players and mp3 players already have an output of around 100+mVolts the line in input is ideal and normally has flat response sound.
Other inputs such as a record player/turntable input has a tailored frequency response to suit record / vinyl only, although it's input sensitivity is similar to the microphone input of around 5mVolts, if a microphone were to be plugged into this input,it would work and you would hear it BUT the frequency(i.e. Bass,mid and treble) wouldn't be a flat response and would sound wrong.
Hope all this info helps somebody