Korean is, in most ways, a fairly easy language to learn,
IMHO. One thing that frustrates newcomers to the language, however, is the existence of two different
number systems.
The first system comes from Chinese, and is easy to use. 1-10 are: eel, ee, sam, sa, o, yook, chil, pal, koo, ship. 100: bek, 1000: cheon, 10000: man. It's easy to say large numbers, because, for instance, 8273 would just be "pal cheon ee bek chil ship sam" (eight thousand two hundred seven ten three).
The second system is Korean, and is a bit harder. 1-10 are: hana, dool, set, net, ta-seot, yeo-seot, eel-geop, yeo-deol, a-heop, yeol. What makes them harder to use is that there are separate words for twenty, thirty, etc. Even more confusing is that for numbers greater than 100, you must use the Chinese numbers for the 100s, 1000s, etc., but the Korean numbers for the 10s and 1s. So 1913 in this system would be cheon ku bek (Chinese 1900) yeol set (Korean 13).
If this wasn't bad enough, one also needs to learn the circumstances in which one uses one system or the other. The Chinese numbers are used for pure numbers: prices, phone numbers, room numbers, shoe sizes, etc. The Korean number are used for counting objects. Oh, and if you're telling time, you use the Korean system for the hours (shi), but the Chinese system for the minutes (boon). So 5:30 would be da-seot shi sam ship boon. Further confusions exist, of course (you use the Chinese numbers if you're using a number in conjunction with another expression than comes from Chinese), but this is enough of an introduction for someone just starting to learn to speak Korean.
See also: hangul, Korean grammar