The cut is the most common type of video transition. It simply means replacing one shot instantly with the next.
When you shoot video footage on your camera, there is a cut between each shot, i.e. between when you stop recording and start recording the next shot. Although some cameras do offer built-in transitions, most recorded footage is separated by cuts.
In video editing and live switching, cuts are fast and efficient. Once a scene has been established, cuts are the best way to keep the action rolling at a good pace. Other types of transition can slow the pace or even be distracting.
Of course there are some situations where fancier transitions are in order. Certain genres of television, for example, rely on a variety of transitions. Even in these productions though, notice how many transitions are still simple cuts.
A common mistake amongst amateurs is to shun the cut in favour of showiness, adding wipes and effects between every shot. Learn to avoid temptation and stick to the basics. The video shots are what the audience wants to see, not how many transitions your editing program can do.