"甲, 乙" are followed by 8 others making a sum of 10 as follows:
甲, 乙, 丙, 丁, 戊, 己, 庚, 辛, 壬, 癸
They are pronounced as :
jia, yi, bing, ding, wu, ji, geng, xin, ren, and gui;
They are called 是天干 (shitiangan) ,which means literally the 10 heavenly stems.
They are used cyclically in the calender constituting the counting system in the Chinese calender. They are also used as ordinal numbers like the Roman I, II, III .
They do have so many applications in Chinese Astrology ( calender ) , Feng shui , Chinese herbal medicine and Acupuncture.
Otherwise you will rarely use them in daily speech or any written context.