Experience point

From WoWWiki

An experience point (abbreviated EXP or XP) is a unit of measurement for character level advancement. Advancing to the next level requires a fixed amount of experience points, determined by a formula.

In World of Warcraft, you generally only get XP from killing mobs and completing quests, but you can also get some from exploring.

This is based on the concept that as characters defeat stronger enemies, they gain combat experience that allows them to improve their skills. This system is descended from D&D and is present in some form or another in almost all roleplaying games.

Mob XP

(This information does not appear to be correct and is undergoing review. For the correct information see Formulas:Mob XP -- Greenman and Gemminie)

When you kill a mob of a certain minimum level, you get XP. That minimum level depends on your current level. There is a formula to calculate that minimum level: take your level, add 1, divide by 5, and round up. If you get less than 5, erase that number and replace it with 5. The result is how many levels below you your minimum experience-yielding mob level is. In short,

MML = CL - max(ceil((CL + 1)/5), 5)

If you'd rather just look it up, here is a table listing the minimum mob level that yields experience for a given character level:

Char Lvl	Min Mob Lvl		Char Lvl	Min Mob Lvl		Char Lvl	Min Mob Lvl		Char Lvl	Min Mob Lvl
1	1		21	16		41	32		61	49
2	1		22	17		42	33		62	50
3	1		23	18		43	34		63	51
4	1		24	19		44	35		64	51
5	1		25	19		45	35		65	52
6	2		26	20		46	36		66	53
7	2		27	21		47	37		67	54
8	3		28	22		48	38		68	55
9	4		29	23		49	39		69	55
10	5		30	23		50	39		70	56
11	6		31	24		51	40
12	7		32	25		52	41
13	8		33	26		53	42
14	9		34	27		54	43
15	10		35	27		55	43
16	11		36	28		56	44
17	12		37	29		57	45
18	13		38	30		58	46
19	14		39	31		59	47
20	15		40	31		60	48

Warlocks probably have to pay more attention to this than anyone else, because, even at level 70, they still have to kill green or better mobs to acquire a Soul Shard.