An analog signal is a continuous wave form. A digital signal is the closest representation of that analog signal in the form of 0's and 1's. DAC's as the name suggest, are converters from one form to the other.
Headphone ports on computers indeed DAC's as they convert 0's and 1's, lets say from an mp3 song perhaps, into an analog signal sent to your earphones where you hear the continuous wave again. The microphone works the same way, just that now it is analog to digital. A microphone generates an analog signal, which is then converted to 0's and 1's by the microphone jack so that the computer can use it for lets say skype, music recording, etc. Remember that the ports themselves are NOT DAC's. But rather in union with the sound card, they are DAC's and ADC's. And DAC and ADC is not entirely different. All that changes is the direction, whether from analog to digital or the other way round.
Some examples:
1) Audio interfaces for home studio recording (Both DAC and ADC) example : M-audio Fasttrack
2) Cable TV - which have components that convert a analog cable signal into a digital image.
3) AV receivers, which convert digital hdmi signal into analog signals to send to speakers.
4) DVD players, convert digital material dvd to analog component video.
5) Most cellphones today use digital encoding. Coverting analog radio frequency signal into digital.
6) fax machines, almost all of them these days are digital. But are transmitted through analog telephone lines.
7) Modems: carry digital data through analog telephone lines.