Charisma measures a character's force of personality, persuasiveness, ability to lead, and physical attractiveness. It represents actual personal strength, not merely how one is perceived by others in a social setting. Charisma is most important for paladins, sorcerers, and bards. It is also important for clerics, as it affects their ability to turn undead. All characters benefit from having a high charisma when speaking with others in the world. Charisma affects an NPC's initial reaction to the player and it modifies the player's persuasion skill checks.
Charisma may be the least important of the abilities, as far as gameplay is concerned. Other than affecting the skills and classes listed above, the effects of this ability are typically restricted to characterizations and roleplaying concerns (i.e. affecting the wording within conversations), with little to no impact on what can be accomplished. Furthermore, the degree to which conversations are affected depends largely upon how much time a writer was able to spend on each conversation. This has led numerous players to "dump" charisma, intentionally leaving it as low as possible so that more points are available for the other abilities in the point buy system.
The core scripts (specifically
nw_i0_plot) define three ranges for charisma — "low" (less than 10), "middle" (10 to 14, inclusive), and "high" (15 or more). Whether or not these ranges mean anything depends on the module being played. (For example, the original campaign has numerous cases where NPCs have different greetings based on these ranges.) The ready availability of these ranges does make these breakpoints likely, if charisma ranges are used at all.
Bard and sorcerer spellcasting is charisma-based (not intelligence-based as players of previous editions of Dungeons & Dragons might assume). This has several consequences. The highest spell level that these classes can learn or cast is the caster's base (unmodified) charisma score minus ten, while bonus spells are given to these classes based on the caster's current (modified) charisma modifier. In addition, the charisma modifier is added to the DCs of spells cast by these classes.