Time-Lapse video is generated by displaying frames faster than you shot them. This gives the visual effect of making things look as if they were happening very quickly. The most ubiquitous use of this effect is in cloud and traffic shots; Clouds roil by as if the wind were moving at hundreds of miles per hour, and cars become little more than a blur heading up the streaks of their taillights. Other popular examples include the bloom and closure of flowers, and industrial uses such as showing the results of materials testing which might otherwise be an extremely lengthy process.
Originally, time-lapse video was created using film cameras which were obliging enough to shoot every second or so rather than, say, 24 times a second. These days it is possible to do it digitally, either in post-processing by grabbing every Nth frame, where the value of N changes the apparent speed of the video, or by only grabbing a frame every so often in the first place.