Parts of a Nappy
Any nappy consists of 3 layers.
1. A liner
2. An absorbent layer
3. A waterproof layer

A Liner
A liner is the layer in the nappy that sits closest to your baby's skin. It can me made of a washable fabric such as microfleece or polar fleece (shown on the left) or a paper-like disposable material. It performs 2 functions in the nappy:
1. It makes cleaning up a breeze. A flushable liner 'catches' any solid matter that can then be simply dropped in the toilet. A washable liner is usually made of either polar fleece or micro fleece. Both of these materials allow most solid waste to just roll off the fabric into the toilet.
2. It keeps your baby's bottom dry. Washable nappy liners are particularly good at wicking moisture away from your baby's skin into the absorbent layers. They are also very soft, breathable and dry against yourbaby's skin. A nappy liner is an optional addition to your cupboard.
Note that fabric softener should not be used on washable liners as this clogs up the liner so that moisture will not pass through it.

An Absorbent Layer
The absorbent layer is the part of the nappy that absorbs liquid. It can either be a fitted absorbant layer such as the fitted nappy shown on the left, or it can be a lay in absorbant layer. This means that you simply lay the absorbent layer (usually a pad or folded square)) inside a waterproof cover. Fabric softener should not be used on an absorbent layer. The following materials are those that are most commonly used:

1. Cotton Flannelette. This material makes a trim, cost effective nappy. It is the least absorbent of the materials used.
2. Cotton terry Toweling. This is the traditional material used for nappies. It is more absorbent than flannelette but does make a bulkier nappy. It can make a cost effective nappy depending on whether the cotton is organic or not.
3. Hemp. Hemp is very absorbent and also very trim.
4. Microfibre. Microfibre is an exceptionally absorbent material. Since it is so absorbent, it is questionable whether or not it should be directly against your baby's skin. It is therefore recommended to cover the microfibre with a liner. Microfibre is also very light, trim and quick drying. It is not a natural fibre.
5. Bamboo. Bamboo is also an exceptionally absorbent and trim material . It also stays super soft after being washed and dried.

Boosters (Doublers, Extenders)
A booster is used to 'boost' the absorbency of a nappy and hence the length of time the nappy can be worn. It is a piece of material designed to sit snugly inside the nappy. A booster is especially useful for nighttime use and for use when you know that your baby's nappy can't be changed for a long time.
Again fabric softener should not be used on boosters as they are an absorbent layer in the nappy.

Note that fabric softener should not be used on absorbent materials as this clogs up the material reducing it's abilty to absorb moisture.

Waterproof Layer (Covers, Soakers)
The waterproof layer can either be sewn into the nappy, or can be purchased separately and used over the nappy - a cover. It is important to choose a waterproof layer that allows your baby's skin to breathe. The most common breathable waterproof fabrics are as follows:

1. Polarfleece or Microfleece. These fabrics are water resistent but not waterproof.They are useful as daytime covers when the baby is moving around and the nappy is changed frequently. If compressed (for example in a baby pouch), they may leak.
2. PUL (Polyurethane Laminate). Laminated and coated fabrics are very popular for use as a waterproof layer in a nappy. Regular fabrics are literally laminated which results in a fabric which is completely waterproof yet breathable.
PUL cannot be soaked in sanitizing solution as this affects the waterproof properties of the fabric. Exposure to vinegar or bleach has the same affect. PUL can be tumble dried on low for short periods of time, however it is a quick drying fabric and it is therefore usually unneccesary to tumble dry.
3. Wool.There are 2 types of woolen covers.  A woven wool fabric is used to make wrap style cover. Regular wool is used to make handknitted pull up covers (soakers).
 Wool covers are most popular for night use. This is because it is a breathable, natural fibre. Since they neutralize odours and are usually used once a day, they can often be used for quite a few days (even a couple of weeks) before requiring a wash. As with fleece, they may leak if they are compressed.
Wool works best if it is lanolized. This is a very simple procedure that requires dissolving a small amount of wool fat (such as Lansinoh) in water, allowing it to cool to lukewarm, and then soaking the wool cover in the water for about half an hour. This is usually done after washing. Wool covers perform best with lanolizing but should still work without lanolizing.
Woven fabrics can usually be machine washed on a wool cycle. Handknitted soakers usually require handwashing. Wool should not be put in the clothes dryer.