Organisms of a class are divided into orders. These orders are based on characteristics listed on a taxonomy key. This key serves as a checklist to see what characteristics an organism shares with other organisms. If organisms share certain, key characteristics, then they are said to be in the same order. Because the taxonomy key is based purely on observation and not science, some might disagree on whether an organism has certain characteristics. For this reason, the organisms within an order are not set in stone.
There are many orders within each class of organisms. Humans are in the Animal Kingdom, the Chordata Phylum, and the Mammal Class. Our order is that of Primates. We are a part of this order because we have an opposable thumb, are highly intelligent, and there is a strong bond between mothers and babies. Other mammals in this order share these characteristics with humans.
Some people believe that when organisms share such close commonalities, as those within an order, there must be a common ancestry among those organisms. Others believe this to be false. There is much controversy and debate on this matter.