The basic leader is a section of monofilament line connected to your fly line; on to which you tie a fly and go fishing.  A leader of this type is called a level leader.  

Level leaders don't roll over and present a fly well.  They tend to collapse into a pile of mono with your fly sitting on top.

Tapered leaders on the other hand do roll over well and so present a fly well. The basic tapered leader consists of three parts. They are: The butt, the middle & the tip.

BUTT

MIDDLE

TIP

To achieve the energy transfer needed to roll a fly over; the leader butt section is matched to the diameter of the fly line.

To pass a modest amount of the captured energy on towards the fly; the middle section tapers gradually.

To present the fly; the tip section (tippet)  is matched to the size of the fly.  This section is used up when changing flies and is easily  replaced.

Generally 60% of leader Generally 20% of leader Generally 20% of leader