For basic purposes, the answers above are correct: lunch is for mid-day, and dinner is in the evening.
However, there are regional differences. Definitely in Northwest England (Eg. Leeds, Manchester, Liverpool) and Ireland, the word "dinner" sometimes refers to the mid-day meal (lunch). Perhaps because there it is often the largest meal of the day, and a smaller supper or tea - "tea" also referring to a meal, not the drink alone - follows in the evening. Note the definitions for "dinner" Richard has found: sometimes the usage is based on the size of the meal, not the time of day it is eaten.
Yes, the locals get confused over this also. ;)