Just something I've always wondered—most sound cards have both a line in and a microphone, and I've never been able to tell the difference between anything recorded with them. Lately I've seen cards without the line in, so is there actually a difference? If so, specifically what should they be used for?
Line in will usually be stereo and about 10Kohm impedance.
Mic in is mono and about 600-1Kohm impedance and expecting SIGNIFICANTLY lower levels than the line, as it has a preamp to pick up the very low levels present in a microphone.
If you have a mic plugged in, and some other input in Line In, your computer's operating system will (or should) use the mic when you're using Skype or something. That way you don't have to do any configuration to have the mic work properly.
On good cards, the mic line will have lower impedance and should be mono. If you try to use it for something else the sound will be distorted.
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
A side note that's probably not so important, but I think of it as Mic Input is for weak signals and Line In is for Boosted Signals like from a Guitar Amp.
(As some have already mentioned Mic into Line-In would be Quiet which can sometimes be favourable if you plan to boost the signal later - but if you put a Guitar Amp signal into your Mic it'll probably not go so well)