Loss prevention is one aspect of security. It is mainly an issue that is faced by retailers, but it can be faced by other sorts of businesses as well. Loss prevention is the process of trying to prevent theft and fraud. Theft and fraud, of course, lead to losses of profit and can be very damaging to a business.
A business has to worry about a variety of types of theft and fraud. For retail stores, there are two main types. First, there is theft and fraud on the part of customers. Shoplifting is a major example of this type of loss. Stores work hard to prevent this sort of loss through things like security details and closed circuit cameras. Second, there is theft and fraud that is done totally or partially by store employees. Sometimes, this occurs when employees simply take merchandise from the store. Other times, it can occur when they help people that they know steal merchandise. For example, they might ring up a few items for a family member and put a number of other (unpaid for) items in their bag as well.
Loss prevention, then, is the aspect of security that focuses on preventing these sorts of theft and fraud.