Hormones are chemical messengers which act in the body to make something happen, or which cause something to change. They usually act in a different place to that in which they are produced. You tend to find that they're actually very powerful chemicals, because they can cause an effect even when they're in the blood stream at very low concentrations.
Hormones are a great way of co-ordinating the response of the whole body because one hormone can affect lots of different tissues. Usually hormones will cause the same thing to happen all over the body, but in fact, hormones are able to have opposite effects depending on which receptor they connect to.
Another important thing to know about hormones is that they're usually very short-lived - they have what is known as a short half-life, which means that they don't last very long. They're produced, thrown out into the blood stream, and then they are quickly degraded or used up or broken down so that they don't usually have they're effect for very long. This means they work extremely well as a signalling molecule.
The endocrine system is an organ system - a group of organs which in this situation is responsible for getting signals around the whole body. There are certain organs which are primarily endocrine (i.e. their main function is producing hormones), but others which have a number of functions which happens to include production of hormones (e.g. the kidney).
The basic structure of the endocrine system includes an endocrine gland, which produces one or more hormones, and an end organ, which receives the hormone and responds in a particular way. The endocrine gland secretes (or releases) the hormone into the veins running away from the organ, and the hormone is then spread through the circulatory system to every organ in the body. Of course, some of these may not respond to the hormone - but some will, and it is at these places that there is a specific receptor, which brings about a response.
There are, in fact, loads of organs which produce hormones. The main endocrine glands (i.e. those which are most important for making hormones) are the pituitary gland, the thyroid gland, the adrenal glands, the pancreas and the gonads (ovaries in women, testes in men). However, the brain, the heart, the kidneys and even fat cells are responsible for making certain hormones that have an important effect in regulating the body's function.
The most important part of the endocrine system is probably the hypothalamic-pituitary axis. The pituitary gland sits just in front of the brain, and is connected to the part of the brain called the hypothalamus by a thin stalk. The reason it is important is because it is from here that many of the other hormones in the endocrine system are regulated - hormones are released from here to stimulate the other endocrine organs to release their hormones.
The two major types of hormones are peptide hormones and steroid hormones.
Peptide hormones are, of course, proteins, and because they are such huge molecules with varying polarity, they can't diffuse easily in and out of cells. It sounds like quite an unimportant detail, but in fact it's key to a lot of the way that peptide hormones work. For a start, it means they can be stored in the cell, and they circulate freely in the blood stream. Most importantly, because they can't get in and out of the cell, they are secreted out of a cell into the blood by exocytosis. When they get to the cell they need to affect, they tend to work on receptors on the surface of the cell because they can't get in so easily.
Like many other chemicals that the body uses, peptide hormones tend to start as a prohormone, which means it's like the hormone with a bit extra added on. This means it is a bit heavier (i.e. a higher molecular weight), and it is made in the same place as every other protein - in the rough endoplasmic reticulum. It is then converted into its proper hormone state (by chopping off the parts of the prohormone which aren't needed) in the Golgi apparatus, before being stored in vesicles. Endocrine glands are made up loads of cells, each of which is involved in producing these hormones. A single endocrine cell can have thousands of these vesicles, all ready to be fired off into the blood stream when the time comes.
Steroid hormones are, of course, a bit different. Unsurprisingly, they're made from the same original chemical: cholesterol. We're often told that cholesterol is bad, and in high levels it is - and unfortunately, we often do find that it is too high in people. However it is also an important chemical that the body needs for signalling. Unlike peptide hormones, it can diffuse very easily across cell membranes, which has an important effect on how steroid hormones are managed. For a start, steroid hormones have to be stored as their precursor (cholesterol) to stop them diffusing out and getting lost (or having their effect too early!).
However, perhaps most importantly, they have their effect inside the nucleus of the cell because they can diffuse right in. This means steroid hormones are able to affect the kind of things that the cell is involved in. By controlling the brain of the cell, they are able to activate or inactivate particular sections of DNA or RNA, completely changing what is going on. Very clever!
Even though steroids are very different from peptide hormones, proteins are still important. For a start, in order to get from one point to another, steroid hormones tend to be bound to a specific protein circulating in the blood stream; this often stops the hormone from being active, and it is only when the hormone becomes free at the other end that it is able to have its effect. This is obviously important to make sure the hormone makes it that far! The other way that proteins are important is in the simple manufacture of the steroid hormone in the first place. A series of enzymes are needed to convert cholesterol into a steroid hormone; if one of them is inhibited, the cholesterol may take a different route and become a completely different hormone. This is important, because it shows that if there is a lack of a particular steroid hormone, it may not be a problem with the hormone itself but because of a problem with one of its required enzymes.