Actors: “Actor is the base class for an Object that can be placed or spawned in a level. Actors may contain a collection of ActorComponents, which can be used to control how actors move, how they are rendered, etc. The other main function of an Actor is the replication of properties and function calls across the network during play.”
Pawn: https://docs.unrealengine.com/latest/INT/Gameplay/Framework/Pawn/index.html
“The Pawn class is the base class of all Actors that can be controlled by players or AI. A Pawn is the physical representation of a player or AI entity within the world. This not only means that the Pawn determines what the player or AI entity looks like visually, but also how it interacts with the world in terms of collisions and other physical interactions. This can be confusing in certain circumstances as some types of games may not have a visible player mesh or avatar within the game. Regardless, the Pawn still represents the physical location, rotation, etc. of a player or entity within the game. A Character is a special type of Pawn that has the ability to walk around.”
Character:
https://docs.unrealengine.com/latest/INT/Gameplay/Framework/Pawn/Character/index.html
“With the addition of a CharacterMovementComponent, a CapsuleComponent, and a Skeletal Mesh, the Pawn class is extended into the highly-featured Character class. A Character is designed for a vertically-oriented player representation that can walk, run, jump, fly, and swim through the world. This class also contains implementations of basic networking and input models.”