Industrial and commercial fixtures are often the ones using PAR lamps, although they will work very well in almost any place where you are using flood and spot lights. Track lighting in a den, kitchen, or living room is an excellent location as well. PAR light bulbs provide bright, efficient, high quality light. Among other things, one benefit of this would be a very high color rendering. This means objects under this type of bulb will reflect a truer, more vibrant light quality than they would under standard flood lights. As a side effect to being brighter than normal reflector lights, the PAR lamp is also about 10% hotter, so they should be used in places where the extra heat won't be a problem.
Consider the task your halogen flood bulbs are doing for you. Are you featuring some decor on a wall with them? Are you highlighting a rack of clothing, toys, or stoneware? Maybe you are using them as a general lighting source for the entire room. Depending on your answer, choose a beam spread that matches your task. General lighting would be a good place for a flood or wide flood bulb, whereas highlighting objects requires the precision of a spot or possibly a narrow flood.
Note: The number after "PAR" indicates the diameter of the bulb in eighths-of-an-inch. You can switch between them anytime, just make sure the measurements match up with the recessed can, track fixture, or other unit that you are using them in.