UPDATE: In research for this small post, I was disappointed to find that there was no wikipedia article on Seohak. So I created one. Ah.. you never forget your first...But it soon turned into a case of "If you give a mouse a cookie...". Once I created that page, I also went on to create a page on the Catholic Persecution of 1801, the founder of Donghak, beefed up a few other related pages and added a few links here and there. It turned into a
Silhak (한글:실학, 한자:實學, "Practical Learning")
(실 = practical, actual) (학 = school of thought, studies)
Sometimes written as Sirhak, this movement was a social reform movement originating after the 1592 Japanese Invasion (임진왜란). Essentially, this reform movement was an attempt at Korean nationalism and a self-identity separate from China's ever present sphere of influence. It gained momentum by appealing to the lower classes who had a lot to look forward with such promised improvements. It's also intrinsically tied to the introduction of Christianity (namely Catholicism) into Korea. Although they were not mutually exclusive, not all Silhakers were Seohakers (yes I just created those terms).
Donghak (한글: 동학, 한자: 東學, "Eastern Learning")
(동 = east)
This "Oriental Culture" movement was deemed a religion but like all religions, many of the precepts are philosophical in nature. The donghaks are noted for cutting their long hair short. Donghak, unlike the other movements, had a distinct leader and founder (Choe Je-u). This movement was established in 1860 and was fiercely opposed to the Seohaks. They still exist today in an evolved form known as 천도교.
Seohak (한글: 서학, 한자: 西學, "Western Learning")
(서 = west)
Many of the Silhakers (yes, there's that made-up term again) were also active in this movement. This was essentially the introduction of Catholicism to Korea. It also brought with it western technologies but was initially dismissed as a Buddhism knock-off. What's important to note is that Seohak represented social and religious change that was considered dangerous. Subsequently, many Seohak believers were persecuted.