aahhh...It is time you start investigating the Trackview
Animation is stored in what is known as "keys".
A key holds the positional, rotational and scale information of an object.
An animation has at least 2 keys, one defining the begin situation, one for the end situation.
You can copy and modify the way keys are blending together (making an ease in/out for exemple).
If you drag the end key to the left, you're shortning the animation, while the total number of frames remains unchanged.
To increase a number of frames, RC on the play button and set the total length to your desires...