Series = sarcomeres in a line:
_ _ _ _ _ _
Parallel = sarcomeres side by side =
_
_
_
_
As my accomplished drawing skills have demonstrated, series results in greater lengthening, so you end up with a heart chamber that has some hypertrophy as well as dilatation (increase in chamber volume ie the amount of blood it can contain). This is an adaptation to increased preload and is called eccentric hypertrophy.
Increased afterload, on the other hand, results in a greater degree of hypertrophy, but without enlargement of the chamber cavity. Hence greater hypertrophy, but without dilatation. This is called concentric hypertrophy.
So,
increased preload = sarcomeres laid down in series = eccentric hypertrophy
increased afterload = sarcomeres laid down in parallel = concentric hypertrophy
