First things first: there' s no point going into detail about pituitary tumours without mentioning the pituitary itself. It's a little gland about the size of a pea, which sticks out of the base of the brain. Here's a nice straightforward picture from Wikipedia with the pituitary gland highlighted in orange:
It basically sits way back in your head, directly behind the very top of your nose, between your eyebrows (unless, of course, you're Frida Kahlo). Even though it looks pretty insignificant, the pituitary gland produces a variety of different hormones which regulate stress, metabolism, growth, reproductive processes and more. It's an important part of the endocrine system and in fact is sometimes known as the 'master gland', so problems with the tiny pituitary can affect the entire body.
Adult pituitary disease is usually caused by a tumour, called a pituitary adenoma. Head to the 'About pituitary adenomas' page to find out more!