Touch attack

A touch attack is an attack in which the attacker must connect with an opponent but does not need to penetrate armor. Touch attacks may be either melee or ranged. The target's armor bonus, shield bonus, and natural armor bonus (including any enhancement bonuses to those values) do not apply to AC against a touch attack.

Attacking with a weapon is not a touch attack, but certain spells such as Ghoul touch and Eldritch blast requires that the caster makes a successful touch attack for the spell to work.

Ranged touch attack

Ranged touch attacks take into account the attackers dexterity in addition to their base attack bonus, as well as any normal attack bonuses (such as from Aid).

Melee touch attack

Melee touch attacks take into account the attackers strength in addition to their base attack bonus, as well as any normal attack bonuses (such as from Aid).

Full calculation

When you are the target of a touch attack, your AC doesn’t include any armor bonus, shield bonus, or natural armor bonus. All other modifiers, such as your size modifier, deflection bonus Dexterity modifier, and dodge bonus (if any) apply normally (or, if flatfooted do not apply with the exception of deflection).

The attack and defensive values are calculated as such:

Attacker: 1d20 + base attack bonus + bonus stat (see above for type) + attack bonuses
Defender: 10 + Dexterity bonus + Dodge AC bonus + Deflection AC bonus+  Size modifier

Generally, it is very easy to hit people relying on high armour, while it is much harder to hit those with high dexterity.