What You Need

Whatever make or type of nappy you have, you will need:

LINER.
Something to draw the wet away from baby's bottom and catch poo, this is usually called a LINER.
Liners lie inside the baby's nappy next to their skin, many nappies will have a fleece liner integrated into the nappy.
Varieties of materials include:
Flushable and biodegradable paper liners
Fleece liners
Silk liners - Not as common as fleece and paper, expensive and difficult to wash.

NAPPY
Something to soak up the wetness. This will be the NAPPY
Nappies can be categorised and each have their pros and cons,
The types of nappies are described in detail on the Product Pages:
FLAT NAPPIES include; Terry Squares, Muslins or Prefolds, these are the cheapest type of nappy and will require some folding.
SHAPED NAPPIES; Many different shapes, sizes and materials with different fastenings.
Shaped nappies are either sized where generally 2 sizes of nappy will take your baby to potty training, or Birth-to-Potty Nappies which are a 1-size nappy taking your baby to potty training.
Materials include; Terry Cotton, Bamboo, Hemp, Brushed Cotton and Micro-terry.
Fatenings include; Velcro (also called Applix), Poppers and Nappy Nippa.
Some shaped nappies will have a liver integrated within the nappy, all will require a waterproof outer.
STUFFABLE NAPPIES; There is a choice of either sized stuffable nappies or birth-to-potty.
Stuffable Nappies have a waterproof outer and a fleecey layer to go against Baby's skin. Between these you stuff the nappy with anything absorbent.
ALL-IN-ON-NAPPIES; Do exactly what it says in the name - a Waterproof outer with an absorbent inner, some also have a stay-dry layer which acts as a liner. The nappies are fitted just like a disposable and the whole lot is washed at every change.

Nappies will vary in absorbency but boosters can be used to add extra absorption to some nappies.

WRAP
A waterproof outer to keep all the wet enclosed, Usually called a WRAP
Materials include; The most common Polyurethane Laminate (PUL)
Fleece and Wool Wraps are also available.
The wrap is fitted over the nappy and fastening is usually velcro or poppers.
An average size baby will usually need 3 sizes of wrap although there are extra-small and extra-large sizes.
Wraps are not required for All-In-One Nappies or Stuffables / Pocket Nappies.

For a complete kit you will need 20/21 nappies and 5/6 wraps, then if you change 7 times a day you will be washing every 3rd day.
Younger babies may need changing more often and you will probably wash more often, it is not worth buying more nappies as you may have trouble fitting them in your machine. As babies are weaned you should be changing their nappies less often.
As a general rule you should not need to change your real nappies any more often than you would change a disposable, If you are you may have something wrong such as the fit of the wrap or the absorbency of the nappy.