Click on your animated object and switch to an IPO-window.
An “IPO” is simply a curve: the horizontal axis is time, and the vertical axis is the value of a single variable, such as LocX. (Obviously, IPOs usually come in groups of three, like LocX, LocY, LocZ.)
When you set a keyframe in an animation, what you are actually doing is establishing a set of fixed-points on the IPOs. The curve will pass directly through these points. For all other frames ("'tweens"), the interpolation of the mathematical IPO curve will determine the variable’s value.
Now, there are three kinds of curve-types, including Beziers and Linear. The default, Bezier, curve will move smoothly between the points, and in so doing it will automatically give you the nice “lead-in lead-out” effects, but in certain cases it will seem to “bounce.” Linear interpolation connects the points with a straight line.
IPOs are the fundamental “fuel” of the animation system; there can be hundreds of them in a simple project.