A Creature is a collection of one or more cells. Each creature belongs to a species. Cells are able to mutate over time as they reproduce.
Each cell is defined by certain statistics and potentially it's relationship to other cells. The statistics are Health, Attack, Defence, Storage, Speed, Max Energy and Fat Storage.
The behavior of each creature is governed by their DNA, which determines the programming of each cell. This programming is hidden in all modes except Laboratory, where it can be edited (See the DNA page).