A graph is a picture of a group of numbers.
Data is a list of numbers that gives you information. Depending on the kind of data you have, you can choose between different types of graphs.
Circle graphs show how a whole breaks down into parts. The graphs show how many of each part are in the whole, OR the fraction or percentage of the whole that each part represents.
If you know the total number of items in the whole, you can figure out the number of items in each part using a circle graph.