Perspective in a picture is dependent on object to camera distance and focal length of the lens used. It changes the way subjects look both in size and apparent distance. With wide-angles the background appears to be much further away from any subject in the foreground than when using tele lenses. Since wide-angles open up perspective while telephotos compress perspective it is difficult to guess actual distances between subjects in an image.
Because of this phenomenon, when you are very close to a subject with your lens, it causes elements in the picture to become disproportionate. For example when taking a close-up portrait with a wide-angle, facial features will be distorted with a big nose compared to the rest of the face. It is easy to correct this kind of distortion by simply moving back and using a longer focal length (50 to 100 mm) on your zoom lens.
When taking pictures of buildings tilting the camera backwards to get everything into the image will introduce converging vertical lines caused by the fact that the CCD plane is not exactly parallel to the image plane. If it is not possible to take a higher viewpoint so you can keep the camera level to fit everything in, this type of distortion can be corrected in Photoshop. By using the Perspective or Skew command in the Edit > Transform menu you can drag the top of your image outward to make converging lines parallel again.