Peer pressure is sort of a pressure from one's peer (someone from the same social,professional group or age category) to behave in a manner similar or acceptable to them.
It is because you identify with them and spend most of your time with them, that this pressure indirectly forces you to adapt their life styles and opinions many times.
One of the disadvantages of peer pressure is causing people sometimes to do bad things they wouldn't do normally such as taking drugs or smoking in the case of adolesence.
Being too long under peer pressure sets norms for you accepted by the group and lets you overlook other possibilites to do different activities or get to know other possibly interesting people outside your group.
You won't be thinking for yourself and many times because you seek acceptance from your peers you won't feel comfortable in your own skin.
These are some of the disadvantages, however there are also advantages such as a sense of belonging , participating in interesting social activities and sometimes being pressured to do some beneficial good deeds,you won't be motivated to do on your own.