Touch range is 2.25 meters. This is meant to be an encompassing range that means giants could touch each other without constantly bumping into each other, or even a human casting Harm on a giant or dragon could do it without constantly walking into the model and not reaching the required distance.
2.25M is actually quite a long "touch" range for gnomes and halflings, and even humans look strange standing so far away to touch a target.
It can bring the caster into attack of opportunity range, although sometimes it doesn't, if casting a hostile touch spell - for instance, Harm.
2.25 meters in feet is around 7 feet.
The rules differ for 3.5E, this is the noted rules on Touch spells:
Touch: You must touch a creature or object to affect it. A touch spell that deals damage can score a critical hit just as a weapon can. A touch spell threatens a critical hit on a natural roll of 20 and deals double damage on a successful critical hit. Some touch spells allow you to touch multiple targets. You can touch as many willing targets as you can reach as part of the casting, but all targets of the spell must be touched in the same round that you finish casting the spell.
Mask of the Betrayer added the Weapon Focus (Melee Touch Attack) and Weapon Focus (Ranged Touch Attack) feats; adding +1 to attack rolls in each case.
It is noted above why there is a short range to touch spells in NWN2.