The hammer is arguably the most used tool in any carpenters toolbox, even with the introduction of nail guns, the hammer is still widely used all over the world. There is also strong evidence which shows that the basic hammer is one of the oldest tools ever used.
The look of the hammer may vary but the basic design is the same, the usual trend being that the hammer will have a head with a flat striking point to it, designed for whichever job the hammer was made for. It also has a long shaft with a gripped handle at the end, these handles can vary in length to be used for the hammer with maximum effect.
Different hammer types are designed for different trades. A carpenter would use a claw hammer, which has a shaft which can vary in length, the longer the shaft the more force can applied to your swing. At the end of the shaft you would find a rubber grip as to stop the hammer slipping out your hands. At the head end you will find a weighted head, one side will have a flat striking point to fix nails and knock timbers together etc, the other end will have a claw to it which can be used to unhook nails out of the timber. Other hammer types can include sledge hammers, pin hammers, framing hammers, wooden mallets, rubber mallets, these types of hammers all have different jobs to do, but can be used in carpentry, usually the claw hammer is the most popular.
Now as the length of the hammers shaft can vary, so can the heads of the hammers, the flat surface can increase in size giving a larger suface area to strike things with, the claw on the other side can flatten out or curve greater, making you able to prize timber apart easier, or to enable you to pull out nails with better effect. Hammers also are made in a variety of materials depending on the job your doing, varying from plastic shafts and heads, wooden mallets, to the more popular steel shafted hammers, which are very hard wearing, which is good for making the hammer last longer.
No matter what the job is, from site work, commercial, or even private work, you can almost guarentee that there are alot of people on that job that will have a trade specific type of hammer to use.