Choose one type of weapon, such as greataxe. You could choose unarmed strike or grapple (or ray, if you are a spellcaster) as your weapon for purposes of this feat. You are especially good at using this weapon. (If you have chosen ray, you are especially good with rays, such as the one produced by the Ray Of Frost spell.)
Prerequisite: Proficiency with the selected weapon, base attack bonus +1.
Benefit: You gain a +1 bonus on all attack rolls you make using this weapon.
Special: This feat can be gained multiple times. Its effects do not stack. Each time this feat is taken, it applies to a new type of weapon. A fighter may select Weapon Focus as one of their fighter bonus feats. They must have Weapon Focus with a weapon to gain the Weapon Specialisation feat for that weapon..
Based on the entry in Player's Handbook 3.5 Page 102.