Barbarians, fighters, paladins, and rangers are proficient with all martial weapons. In NWN a character cannot wield a weapon they do not have the proficiency for.
Some martial weapons are: Longsword, Scimitar, Rapier, Warhammer, Halberd, Falchion. For a full list, see here.
D&D comparison Edit
In DnD 3.5 a character takes a -4 penalty to attack rolls when using a weapon she/he is not proficient with.