Time-lapse can be shot with a timer, which is an external input you can add to your DSLR. (Or camera device, as mobile phones and some other DSLR's have time-lapse equipped into themselves) and you can set the timer to take a picture every 10 seconds for example.
You can also vary the effect of the passing of time, utilising the shutter speed on DSLR's. For example when the shutter speed is low and your doing a time-lapse of a motorway for example, you'd see the car lights follow; but if you were to do the same time-lapse but with the shutter speed higher, you wouldn't get that effect but you'd get clear images of the cars passing by through each snap.
An example of a timelapse would be here:
Timelapse over a horizon on a rooftop
https://www.youtube.com/watch?v=63ZxJCjc1SA