From BlenderWiki

Animation

Animation is making an object move or change shape over time. Objects can be animated in many ways:

Moving as a whole object 

Changing their position, orientation or size in time;

Deforming them 

animating their vertices or control points;

Character Animation via Armature 

animated to deform by the movement of bones inside the mesh, a very complex and flexible interaction that makes character-shaped objects appear to walk and jump.

In this chapter we will cover the first two, but the basics given here are actually vital for understanding the following chapters as well.

Three methods are normally used in animation software to make a 3D object move:

Key frames

Complete positions are saved for units of time (frames). An animation is created by interpolating an object fluidly through the frames. The advantage of this method is that it allows you to work with clearly visualized units. The animator can work from one position to the next and can change previously created positions, or move them in time.

Animation Curves

Curves are interpolated from keyframes, and can be drawn for each XYZ component for location, rotation, and size, as well as any other attribute in Blender. These form the graphs for the movement, with time set out horizontally and the value set out vertically. The advantage of this method is that it gives you precise control over the results of the movement.

Path

A curve is drawn in 3D space, and the Object is constrained to follow it according to a given time function of the position along the path.

The first two systems in Blender are completely integrated in a single one, the F-Curve system.

In Blender 2.5x, everything can now me animated. Previously, only certain datablock had the ability to be keyframed. Now users have the ability to animate nearly any type of data that can be changed to multiple values.

Chapters

General Principles and Tools

Keyframes

Animation Editors

Using The Timeline

Markers

The Graph Editor

Using the Graph Editor

F-Curves

Editing Curves

F-Curve Modifiers

The Action Editor

Actions

Creating Actions

Animation Techniques

Constraints

Moving objects on a Path

Changing Object Layers

Game Engine Physics Recording

Animating Deformation

Methods of deformation

Shape Keys

Absolute Shape Keys

Deforming by a Lattice

Deforming with Hooks

See also Hooks - Uses a modifier as a way to change the shape of a mesh. Sorta like sticking a fish hook in a mesh and pulling. Uses the principles discussed in Shape Keys.

Drivers

Drivers

Driven Shape Keys

The BSoD Introduction to Character Animation tutorial is a good starting point for learning character animation. Even if you never used Blender before.