If an object gains an electron it will become negatively charged and object which looses an electron, it will become positively charged. When they do not come in contact, charge will remain them as it is but they can take or loose charges from the objects.

But when we rub comb from our hair, it attracts bits of paper. After some time the charge is gone . How?

If two objects are not in contact, the charges will remain as it is if they are suspended by insulated stands. In this case charges will not flow to the ground.
When we rub the comb with dry hair then electrons are transferred from dry hair to comb. As electrons are of negative charge hence comb acquires a negative charge and the dry hair become positive i.e. there is transfer of charge from dry hairs to the comb.
In case of a plastic comb, the charge stays on the comb even if we hold it because plastic is an insulator.
In case of conductors, charges can't stay on the object. It is removed by touching the charged regions. Doing this, the charges flow through our bodies to ground.