A shift-tilt lens (also known as a PC, perspective control lens) has a variety of functions but is mostly thought of for correcting distortion on architectural photos. In this segment, Brian Osborne is onsite at a client’s hotel property and actually demonstrates the shift function of the lens in the video.
Go out with a wide-angle lens and point up at your house or a building. Notice how the more you point the lens up the more the vertical lines on the building lean in towards the top. Take a shot like this and then go into your editing software and find where you can correct vertical distortion. This is a great way to learn how much extra space you have to leave if you plan on fixing distortion in post processing. Also, take a photo with the lens pointed straight ahead at the same building and while you might get more ground in the photo than you would like, you will see how the building is much straighter. The final solution is to backup from the building and/or get the camera up higher (on a ladder) to reduce the upward angle of the lens and therefore, the distortion.
While the effect of the distortion is more easily noticed in buildings, the same thing takes place out in nature as well. The example below shows the without and with effects of a tilt-shift lens in the situation of a tall waterfall.