Touch attacks are spells attacks - where you only need to succeed in touching the opponent to inflict damage.
The target's AC is generally lower than against physical attacks because AC from armor, shield and natural AC (base and enhanced) is ignored.
Your AC against a range attack is your Touch AC = 10 + Dexterity Bonus + Deflection AC + Dodge AC
Melee Touch Attack[]
A Melee Touch Attack is one that required the caster to be close enough to touch the opponent. Spells such as Ghoul Touch and Inflict Wounds require a successful melee touch attack.
The casters Strength Modifier is added to the attack roll
Ranged Touch Attack[]
These work over a greater distances and include Melf's Acid Arrow and Scorching Ray.
The casters Dexterity Modifier is added to the attack roll.