Internal. . external. . .
Oh well, here it goes;
Internal martial arts, or IMA, are styles that focuses on the development of chi, ki, prana, pranh(MT) or also known as internal power. Training will be a mixture of fighting techniques and "chi cultivation".
For example, in Bajiquan and Hsing-yi you will most definitely be standing in horse stance for a very long time. It's to build up chi.
Chi is hard to explain, and being novice in it's cultivation, I can't really define what it is but some says it bio-electricity. All I know it's a good replacement for "brute strength", but it also has to do alot with body mechanics.
External Martial Arts, or EMA, you can see all around you. Muay Thai, Shaolin Kempo, Wing Chun, Karate, Tae Kwon Do, etc etc. They are arts that relies on your muscles to do the job and as well as proper body mechanics.
Ask Fisherman or ButterflyPalm about this, they are a lot older than me, have more experience and can possibly kick my arse using the famous "chi balls".