This affect occurs when your camera is not parallel with the subject. In the above example the subject is the building's walls, which presumably are vertical. The camera however is pointing upwards and is therefore not vertical and not parallel with the walls.

You can sometimes solve the problem by pointing your camera straight ahead and not upwards. But of course with most buildings you will chop off the top, and in all cases you will create a large foreground area which will need to be filled with something interesting.

The cure is simple in theory but often difficult in practice. You must elevate your position.

For example, if you're standing at a spot that's about half as high as the subject building then you can point your camera straight at it. All the walls will be vertical and you'll still get the entire building in the photo (subject to having a wide enough lens).

Naturally all rules can, and indeed should, be broken at times. If you can't get the walls straight then go the whole hog and make them really lean. It often produces a very dramatic affect.

NOTE: There are special lenses and cameras that have features that fix this problem, but that's outside the scope of these short tutorials.