A pie chart is an area enclosed in a circle, where the total circular area equals 100% of the unit being measured. A pie chart may be sub-divided into non-overlapping sectors where the area of a sector represents a fraction (or percentage) of the whole unit being measured. These sectors are typically categories of the whole unit being measured.
For example, if you wish to represent the number of fruit in a bowl as a pie chart where there are 18 pieces of fruit, comprised of 6 apples, 9 oranges and 3 bananas, then the following is true:
The total area of the pie chart represents 18 pieces of fruit, or in other words, 100% of the fruit in the bowl.
The pie chart is sub-divided into 3 sections, one section each per type of fruit in the bowl
The sector area of the pie chart representing apples would comprise 1/3 of the pie chart because 6 apples/18 fruit is 1/3. The sector of the pie chart represented by apples is measured as an arc of 120 degrees (or 1/3 of 360 degrees of a circle)
The sector area of the pie chart representing oranges would comprise 1/2 of the pie chart because 9 oranges/18 fruit is 1/2. The sector of the pie chart represented by oranges is measured as an arc of 180 degrees (or 1/2 of 360 degrees of a circle)
The sector area of the pie chart representing bananas would comprise 1/6 of the pie chart because 3 bananas/18 fruit is 1/6. The sector of the pie chart represented by bananas is measured as an arc of 60 degrees (or 1/6 of 360 degrees of a circle)
So, the total degrees of arc for all sectors of the pie chart equal 360 degrees (a full circle).