well basically, there's only a handful of internal martial arts, three to be exact. they include (names shortened) Bagua, Tai Chi, and Hsing-I. (note that since the translation between Chinese and English results in sounds, and not literal words, the names of the martial arts can be spelled differently, according to how they sound when the Chinese word is pronounced. ex: Bagua could be spelled PaGua, Pa Kua, Bagua, etc.). Anyway, so there's those three styles of internal, and literally dozens of external. some of the more common, well known ones include karate, taekwon do, kung-fu styles, jiu jitsu, etc. etc. etc.
External styles generally rely on force, Slow, gentle, calm, etc are the main characteristics of the internal styles. In other words, the external stylists train hard to increase their muscular strength while the internal stylists, their qi (life energy) and jing (internal strength). However, it's unlikely for an art to be classified SOLELY as internal or solely as external, many martial arts have a calm, mediation side of them, and likewise, many internal arts (Tai Chi for instance with its push-hands) have external aspects. Roughly speaking, the difference between internal and external styles can refer to whether the strength is from the torso and legs (internal) or whether the strength is derived from training of the more specific arm and leg muscles (external); whether the strength is primarily mental, or primary physical. The word "internal" often connotes a more pliable martial style.
External styles are generally fast-paced and quick to see results. A good way to think of it is thinking of a well-performed roundhouse or crescent kick of Taekwon do. fairly simple to learn, slightly complicated to master, very easy to use. however, an internal style may be practiced for 10 years before the practicer can "fight". this is because the internal arts aren't dedicated to fighting, as external are. Fighting is just an
aspect of internal arts. other aspects include working with the mind, the qi (chi), etc. When mastered, the internal arts are extremely powerful and effective, in fact they're the most effective, but dedication is vital, since many take martial arts expecting to be able to win fights, and such, internal is not about that. it's a completely different system
