The dermaroller head is rolled over the skin, whereas a dermastamp does not roll and the needles are "stamped" into the skin through manual pressure.
A dermastamp is useful for more targeted and intensive treatment such as for individual acne scars, surgical- and other scars, stretch marks, individual wrinkles, individual hyper- or hypopigmented spots etc.
A dermaroller is suitable for the overall treatment of larger areas such as lost elasticity of the face, neck, décolleté and other areas, for enhancing the penetration of skin- or hair products, for the general improvement of skin texture etc.
Often a combination of the two is very useful, such as stamping individual acne scars and then rolling the entire area, stamping stretch marks and then rolling the entire area, stamping individual wrinkles and then rolling the entire face etc.
The remainder of your questions are answered in our dermarolling instructions: http://dermaroller.owndoc.com/dermaroller-instructions.pdf