Hard vs. Gentle, External vs. Internal
The concept of hard/gentle and external/internal is not one easily described. In terms of styles which most people are familiar with, Karate would be an example of a hard style and Aikido or Tai Chi examples of soft styles. A hard style is generally considered one where force is used against force; a block is used to deflect an incoming strike by meeting either head on, or at a 90-degree angle. A gentle style does not use force against force, but rather deflects the incoming blow away from its target. There are uses for both hard and gentle techniques. A practitioner may wish to break the attacker's striking arm with the block. On the other hand, a much smaller opponent would not be able to accomplish this, so instead may wish to deflect the incoming attack.
An external style is one, which relies primarily in strength and physical abilities to defeat an opponent. In contrast, an internal style is one that depends upon ki and timing rather than power. Aikido (at the master's level) would be an internal style, while most karate styles are external. with the exception of Goju Ryu Karate.
However, the concepts of hard/gentle internal/external are finding fewer proponents among senior martial artists. Both conceptual twins are impossible to separate in reality, and masters will generally acknowledge that any distinction is largely only a matter of subjective interpretation. Novices and philosophical dilettantes, ignorant of the inseparable nature of duality, often wage arguments about the reality of the concepts. They see yin and yang as elements that can exist independently, while philosophical and physical reasoning demonstrate that they cannot. Without their union (Tao), neither can exist. Ergo, a "hard" technique such as a straight fist is guided by the gentle power of mind and the internal component of ki. equally; the gentle projection of Aikido requires the "hard" element of physical contact and movement, coupled with actively redirecting the opponent. In short, preoccupation with distinguishing gentle from hard is a distraction form learning martial arts and moving towards a unifying technique and mastery.