Strength is obviously important for fighters and is considered the prime requisite for that class. A score of 9-12 is average. There are advantages with scores 13 or higher when it comes to accumulating experience, as explained later, so any character with a strength of 13 to 18 should be considered for the fighting class.
Intelligence is the prime requisite for magic-users. Since there are advantages to the magic-user if his intelligence is 13 or higher, any character with such a score should be considered for this class. High intelligence also allows any character to learn additional languages.
Wisdom is the prime requisite for clerics. Clerics can perform miraculous spells even though they do not have special intelligence, and second level (experienced) clerics can heal wounds. If a character has a high wisdom score (13 and over) he would do well as a cleric.
Constitution is a combination of health and endurance. It will influence how a character can withstand being paralyzed or killed and raised from the dead, etc. In addition, dwarves and halflings have strong constitutions and a character should have a constitution greater than 9 to be a dwarf or halfling.
Dexterity applies to speed and accuracy. Characters with high dexterity can get off the first arrow, throw the first spell or draw a weapon and strke the first blow. Dexterity is the prime requisite for thives, and scores of 13 and over in this ability make the character a good candidate for the thief class. Halflings are also noted for their dexterity with weapons and a character should have a dexterity greater than 9 to be a halfling.
Charisma is a combination of apperance, personality, appeal and so forth. Its most important aspect is leadership. A character of charisma below 13 can not hire more than 5 followers, and their loyalty will be luke-warn at best - that is, if the fighting gets hot there is a good probability that they will run away. On the other hand, someone with a charisma of 18 can win over a large number of followers (men or monsters) who will probably stand by him to the death. Also a female with a high charisma will not be eaten by a dragon but kept captive. A charismatic male defeated by a witch will not be turned into a frog but kept enchanted as her lover, and so forth.