Pie, donut, funnel and treemap
Composition, stages and hierarchy.
Visuals for showing how something divides up.
Pie and donut
Parts of one whole. Effective with a few slices and poor with many — past about six, the small slices become indistinguishable and the labels collide.
A donut is a pie with the centre removed, which reads slightly more cleanly and leaves room for a total in the middle.
If the point is "which of these is largest", a bar chart answers it better. Use a pie when the point is genuinely "these sum to one thing".
Funnel
Stages in a sequence, each narrower than the last — visitors, signups, trials, customers. The value is in seeing where the drop is, so order the stages by the actual process, not by size.
Treemap
Nested rectangles sized by value. Handles far more categories than a pie without becoming unreadable, and shows hierarchy — categories containing subcategories. Good for "what is big here" across a long tail; poor for precise comparison, since people judge area imprecisely.