A typical movie is shot and projected at 24 frames per second.
The two main video standards in the US (NTSC) and in Europe (PAL) use different frame rates (29.97 frames per second vs. 25 frames per second), which usually means that the film has to be sped up when transferred to video to avoid image synchronization problems.
In the case of a PAL transfer, this acceleration results in a 4% decrease in the film's total running time. In other words, the European VHS tape or DVD of a 100-minutes theatrical release will only be 96 minutes long.
Other changes in runtime may be caused by cuts or alterations imposed by local censorship rules or other minor changes between different releases of the film (such as shorter/longer end titles, distributor's logos, copyright notices and various supplemental material).