In your example, the words 갑(甲) and 을(乙) are used in general sample names, especially in legal documents, like the alphabet A and B in English. We called them 십간(十干) or 천간(天干, the ten heavenly stems). The ten heavenly stems(천간) include "갑(甲), 을(乙), 병(丙), 정(丁), 무(戊), 기(己), 경(庚), 신(辛), 임(壬), 계(癸)" You can reference more detail infomation about it through the following link. http://en.wikipedia.org/wiki/Celestial_stem Thus, you can say it like this. A가 B보다 낫다. B가 A만 못하다.

What do 을 and 갑 mean? Actually I can understand the grammar structure but did not find the right meaning of 을 and 갑 in none of online vocabularies. Plus there is no context, phrases are given as example for one grammar rule.