Heat map
A heat map is a graphical representation of data where the values taken by a variable in a two-dimensional map are represented as colours. A very similar presentation form is a Tree map.
Heat maps are typically used in Molecular Biology to represent the level of expression of many genes across a number of comparable samples (e.g. cells in different states, samples from different patients) as they are obtained from DNA microarrays.
Heat maps are also used in places where the data is volatile and representation of this data as a heat map improves usablity. For example, NASDAQ uses heat maps to show the NASDAQ 100 index volatility. There are also heat maps for showing the housing price in various regions.
References
Eisen, M.B., Spellman, P.T., Brown, P.O. & Botstein, D. (1998). "Cluster analysis and display of genome-wide expression patterns". Proc. Natl. Acad. Sci. USA. 95 (25): 14863–14868. PMID 9843981 doi:10.1073/pnas.95.25.14863