Help with product description.

Help with product description.

Hi, this is drive me crazy... im trying to add a field of description on a produc, so, anytime i add a product i can be able to write a description. Actually, i only can add a unmodiffy description or a general. for example. on invoice... i want to add a produc like....

qty     Description                                                  price        total
 10     apples and anything i want to write          10               1.00
  5      Photos i take from Spain in holidays         5              25.00

and so on...... allways the description is different.

Thanks.