The difference between external and internal chinese arts is actually pretty minimum. The difference is in their approach. Here is a real simple way of saying it.

External styles usually are led by an internal path. The alignments still have to be correct, but you are maintaing strength (external, hard) on the outside, and calm (internal, energy flowing) on the inside.

While internal arts are soft (this is a misconception, you are just "sensitive") and fluid on the outside, but inside the energy is raging and so is the intention. Internal development is just stressed alot more from the beginning, than it is in external ones, in which you learn apps etc. right away.

But in reality, they are not very different at all, most "external" gong fu, had internal principles. That IS what Shaolin gong fu is, its actually considered internal because of its stress on fluidity, congruity of mind, body, soul, and Zen. Only difference is Shaolin thought the bodies should be in great shape to have the blood, and energies within the body flowing so they would not be lethargic when they meditate.
Truth comes from the absolute stillness of the mind...