Your penis has two main functions – to pee and to have sex.  It has two main parts, the head, and the shaft.  The head of the penis, which can also be called the glans is more sensitive than the shaft.  There is also the foreskin which usually covers the head of the penis.  When the foreskin is removed in some men this is called circumcision.  This may be done for cultural, religious or simply for hygiene reasons.  Your foreskin should roll back when you have an erection. 

Occasionally the foreskin is very tight and doesn’t move easily.  It may stretch in time or you may need to be circumcised to assist this.

Normally a man’s penis is soft and hangs down.  When you get an erection your penis gets stiff, it will grow longer and harder and stick outwards and upwards away from your body.  This is because your penis is made from lots of blood vessels, like a sponge and when they fill up with blood, they expand making it stiffen.  There are no muscles in your penis which is why it’s difficult to move it when you have an erection.  It’s also difficult to pee when you have an erection because the nerve connection to your bladder is stopped for this time.