Receptors are basically the way in which the body's signals are received. The body has many different ways of communicating particular messages, but in each case it requires some form of receptor. These will often (though not always) be on the surface of a cell, and they will cause a particular reaction or change that has an effect in that cell.
They generally require some kind of chemical which the body has produced to bind to the receptor, causing some kind of change which has an effect.
There are different kinds of receptors, including ionotropic and metabotropic receptors.
Ionotropic receptors involve ion channels, so binding of a substance to the receptor opens a channel, allowing a particular ion or group of ions to go through the membrane. This could be going into the cell (e.g. sodium), or leaving it (e.g. potassium) depending on which side of the membrane has the greater concentration of the ion (i.e. the diffusion gradient).
Metabotropic receptors involve some change in the processes of the cell - the 'metabolic processes' of the cell. This could lead to a variety of things, but a common type of metabotropic receptor is the G-protein coupled receptor.
All in all, receptors are very important because nothing much would happen without them! Usually proteins, they have a specific shape which means they will only bind to one particular chemical or group of chemicals. They're a bit like locks; given the right key (the chemical), you can open them up and cause an effect.
Unfortunately it's not quite that simple. Receptors bind chemicals with affinity, and cause an effect if that chemical has efficacy.
Affinity is the ability of a particular chemical to bind to a receptor.
Since receptors have a specific shape, they will not just bind anything - and this is useful, because there are many receptors on each cell, with many different effects. The idea is that a particular chemical has a particular effect, so making sure that not all the receptors will bind is important.
Affinity could be measured in terms of KD, which is the concentration of a particular chemical or drug that is needed for half of the receptors to be occupied in equilibirum. 'Equilibrium' is included because when chemicals bind to receptors, they usually dissociate again. When we talk about half the receptors being occupied in equilibrium, we mean that, at any particular point, given a particular concentration, half of the receptors will have the chemical bound to them. Obviously if the chemical is better suited to the receptor (i.e. it has the right shape), a smaller concentration is needed to get the receptors filled up.
Efficacy is the ability of a chemical to have an effect when it binds to a receptor.
Just because a chemical has the right shape doesn't mean that it's actually going to achieve anything. You can put the plug for your toaster into a socket, but unless there's some electricity there it won't achieve anything. So an important thing to remember about receptors is that the chemical must have both affinity and efficacy - it must have the right shape, and it must be able to have an effect.
Efficacy may also be used to describe effectiveness of something which has no relation to receptors. It is a handy word as it describes how useful something is. It is important, however, not to mix up efficacy and potency. Potency is how powerful a particular drug is - that is, if you have a particular concentration, how much effect will it produce. Efficacy is independent of concentration - it's basically saying that assuming the concentration is high enough for the chemical to be bound to the receptor, regardless of how high that needs to be, will the chemical produce an effect.
I suppose one way to look at it is: efficacy says if you punch me however hard, will it hurt? Potency says how hard do you have to punch me for it to hurt? Affinity says are you able to punch me, whether it hurts or not?
An agonist is something which binds with both affinity and efficacy.
They may be split into full agonists and partial agonists (with a sliding scale between them) - i.e. some may produce the maximum response with only a few receptors activated (full agonists have high efficacy), while some may need to occupy every receptor available and still not reach the maximum response (partial agonists have low efficacy).
Agonists are often described when talking about drugs to refer to drugs that are used which mimic the effects of a natural chemical - for instance, dopamine agonists act on dopamine receptors to produce the same effect as dopamine would.
An antagonist is something which binds with affinity but no efficacy.
They also impair the ability for anything else to have an effect. This could be competitive, where the antagonist is occupying the binding sight for the agonist and therefore preventing anything else from having an effect. It could alternatively be non-competitive, where it's not getting in the way, but it causes a change or an effect on the receptor that prevents anything else binding, or if anything else does bind, it stops it actually achieving anything.
Antagonists are often used when talking about drugs to refer to drugs which are used to prevent the effects of a natural chemical - for instance, dopamine antagonists act on dopamine receptors to prevent dopamine (or dopamine agonists) from having an effect.