Forward covers

Dear members,

The company is involving me in forward covers.

I dont have much idea practically.

Can u pls tell me what is margin, premium, discount (that is added/substracted to the spot rate). And on which factors these things vary ?

Also, wen we take spot conversion, there is some addition/substraction to the spot rate depending up on the delivery period. Pls explain me this. How is delivery period is related to variation in spot rate ?

Pls pls reply...

Thanks in adv..