A pie chart is a circular chart divided into sections. Each section is proportional to the quantity it represents. Also, a pie chart contains a single series of data points.
It is best to use pie charts to show part-to-whole relationships or data composition. Pie charts are not intended for comparing individual sections with each other, or representing exact values (you should use a serial chart for that). Pie charts are also not meant to show more than 7 or 8 data points.
A donut chart is an alternative representation of a pie chart. It can be thought of as a column in a stacked bar graph curled around. Donut charts are created by increasing the value for the Inner Radius (%) setting in the Chart tab of the pie chart element.