Answer:
Depending on your readership and/or visitor, one can classify the advertising. For example FreeJi.ca has classified advertising in following categories.

Buy & Sell
Jobs
Pets
Real Estate (Rent)
Real Estate (Sale)
Community
Automobiles
Services

Each category is further divided in 5 to 15 sub-categories.
First answer by Freeji. Last edit by Freeji. Contributor trust: 0 Question popularity: 4 [recommend question].