A mitochondrion is an organelle for cellular respiration, so produces ATP.
Mitochondria are membrane bound organelles; they have a double membrane structure.
Figure: Parts of a mitochondrion.
Figure: Another picture.
The inner membrane bounds the matrix.
The folds of the inner membrane increases the surface area, allowing more places for the electron transport chain (ETC) (where most ATP is produced) to be active.
The cristae is the folded parts in the middle line of the inner membrane shape. They are in the middle because physically better to operate the ETC.