Creature types, or creature subtypes, are designations given to certain cards based on the creature they represent.
These types can be referenced in certain cards, and therefore create a Tribal connection with like-designated cards. Creatures of the same type oftentimes work better together, while some creatures are designed specifically around interactions with another type. Additionally, there are many spells that limit their function to only interact with creatures of a certain type, and vice versa.
Additionally, most creature cards have a "Race/Class Model". A "Race" implies a species in which the creature belongs, while a "Class" refers to a profession or 'calling'. An example of a Race would be "Elf", and a class would be "Wizard". As such, a combination "Elf Wizard" is oftentimes seen. However, not all cards follow the Race/Class Model; particularly, non-sentient creatures generally don't have a "class".
Alphabetical List of Creature Types