I wouldn't get too hung up on the terms, which is all too easy to do. Some things that I've gleaned off various threads on various boards, is that
1) so called internal seems to be simply good posture, efficient movement, etc., even in the best descriptions from internal theorists
2) some internal theorists say that even external arts have internal stuff in them
3) some internal theorists say that internal strength is simply a training method
4) some think when internal strength is used it would look like regular ol external stuff
If you put 1-4 together, coupled with the fact that there is no test to say if something is internal or not, even the so called experts can't make any real distinction.
Besides claimed different types of strength or the intrinsic vs. outside forms distinctions, another school of thought is that internal refers to martial arts created in China (taijiquan, for example), while external refers to martial arts originally created outside of it (shaolin, for example).
At least the latter has a fairly objective definition, is not too debatable, and is supported by how 'nei' (internal, http://zhongwen.com/d/164/x186.htm
) and 'wai' (external, http://zhongwen.com/d/165/x126.htm
) are used everywhere else in the Chinese language.