The weight of a hammer head is directly correlated to the force it imparts on the object you strike. A heavier head is used in applications where greater force is required i.e. a sledge hammer. When delicate, lighter work is required the hammer head is generally much lighter i.e. an upholstery hammer.
With hammers the material the grip is constructed of has a role to play in the ergonomics, aesthetics, and functionality of the tool. Selection of the grip is largely a matter of personal preference although there may be applications where one type is generally preferable to another.
The shape of a hammer claw determines the leverage you can apply when using it to pry objects apart. Curved claws give you greater leverage and are great for pulling nails while straight claws are more effective for prying larger pieces of material.