Titan weapons are cosmetically the same as normal weapons but do far more damage. They are expensive though and can only be purchased once a normal weapon is upgraded to V.5. Once a normal weapon is converted to a Titan weapon it goes to V.6 and can be fully upgraded to V.10 for maximum damage. If your weapon has mods on it before it is a Titan weapon it keeps those mods and you are still able to purchase any mods available for that weapon.