What does N:P:K stand for, and is it a four-letter word?

I'm asked this often, so here is the short answer:

N : Nitrogen (Good for growth of foliage)
P : Phosphorous (Good for roots and flowers)
K : Pottasium (Good for fruit & general health of the plant)
Four-letter word?: Yes & No

The numbers (e.g. 3:1:5 or 2:3:2) that you see on a bag of fertilizer represent the proportion of these 3 elements - N:P:K.

Some quick facts:

• A lack of Nitrogen is usually quite apparent when the green foliage of your lawn or plants becomes pale. (Although this is not the only reason for pale leaves)
• Phosphorous does not move through the soil, so it should only be added in small amounts near the roots of plants, so that it can be absorbed easily.
• Potassium deficiency shows up when the edges of leaves and the area between the veins start to go yellow. Potassium helps plants handle changes in temperature.
• Generally speaking, unless the fertiliser is slow release (it will have (SR) after the N:P:K) you should always water your plants straight after applying in order to prevent any burning of the plants, and to help them to absorb the nutrients easily. Wash your hands immediately for the same reason.
  
• The plant family Fabaceae (e.g. Peas, Beans, Acacia, Indigofera, Crotalaria) has a symbiotic relationship with bacteria which actually helps add Nitrogen to the soil naturally.
  

But good-old-fashioned granular or chemical fertilizer is poo-pooed (sorry I couldn't resist that) in many circles these days, rather there is a strong move towards using organic fertilizers instead.

The problem with this particular type of fertilizer has resulted from its over-use, and mis-use. Chemical fertilizers are sometimes applied in larger quantities than can be absorbed by the plants or held by the soil, they then leach down into the groundwater and rivers, and can result in the death of fish amongst other things.
It is also believed that in large quantities over time, they can actually poison the soil and kill off the natural organisms that are essential for plants and organisms in the soil.

My personal opinion is that chemical fertilizers should always be just a very small portion of the food that we provide for our soil and plants. Because the elements are in their basic form, and therefore easily absorbed, they are often great as a short term solution.
But organic fertilizers, such as composts and manures, provide a whole host of other macro and micro nutrients, as well as improving the structure of the soil. For these reasons, they are always better in the long run.