Explain me how I use 'Trap' gem

There is a gem named 'Trap'.
It never work with only melee weapon?
It works with spell cast gem or melee attack gem or ranged attack gem or ranged weapon or melee weapon or spell weapon? Please tell me examples how I use 'Trap' gem. What is 15% more trap damage? I am not able seeing any difference with this 'Trap' support gem and with other trap gem.

Trap is a support. You have to link it to a active skill to use it.

Trap supported skills will not work when you are wielding a melee weapon. By extension, trap cannot support melee skills.

Trap causes the supported skills to do 15% MORE damage if the damage is directly from the trap. That means that it is a straight multiplier to the damage that cannot be adjusted. This value will scale with gem level if i remember correctly. If you use Multiple Traps with Traps for example, the skill will still gain a 1.15x multiplier. However, it will then interact with Multiple Traps's multiplier of LESS for a net reduction in damage. Note that REDUCED and INCREASED modifiers behave differently.

Trap will not amplify damage if the damage is not DIRECTLY from the trap. If a trap is used to summon a minion or totem, the summoned unit is attributed the damage. Thus, the 15% MORE trap damage will not affect it. Note that burning for example is still attributed to be caused by a trap if the trap caused it with direct fire damage.

Other trap gems come with an inbuilt spell. Fire Trap is an ACTIVE spell, Lightning Trap is an ACTIVE spell. These gems cannot be used to support skills, they themselves being skills.

A simple example of Trap is using it with Cold Snap. When you throw the trap, it will lay dormant until being triggered or it times out. If triggered by an enemy walking on it, it will cast Cold Snap. As the trap is the one casting Cold Snap, it gains the modifier '15% MORE trap damage' as it is trap damage. The Cold Snap casted will thus have 1.15 damage than if you were to cast it yourself.