There are many different fields of photography, photography can be for domestic or commercial uses, and advertising photography is one area of this. This is on the commercial side and is intended for businesses.
One of the most common places you will see this used is the fashion side, have you ever walked into clothes stores and been draw into the photos around the stores of people wearing that stores brand of clothing? Well that is it at its best.
Models will be required for this work, they will be hired by the company and go on a photo shoot. Depending on the size of the company will depend on what type of photographer they get sent (ie an expensive photographer with a reputation in his field or a relatively inexperienced, cheaper photographer). The model will be challenged with striking a good pose for the camera showing of the companies clothing in all their glory. The photographer will choose the best images, put them forward to the company, in which if they are liked they will be purchased for a price and used for the companies advertising.
This is only an example of Advertising Photography though, in a non-technical way it could be photos of anything at all as long as they are being used in an advertising sense. For example: a photograph of a dog to go on your side at home for personal pleasure can not be classed in this category, however the exact same photo of that dog but used by lets say a breeder to advertise the dog for sale can be classed in this category.