1. Rasters are made up of a matrix of pixels (also called cells), each containing a value that represents the conditions for the area covered by that cell. A raster dataset is composed of rows (running across) and columns (running down) of pixels. Each pixel represents a geographical region, and the value in that pixel represents some characteristic of that region.
2. A raster consists of a matrix of cells (or pixels) organized into rows and columns (or a grid) where each cell contains a value representing information. The cell values represent the phenomenon portrayed by the raster dataset such as a category, magnitude, height, or spectral value. Cell values can be either positive or negative, integer, or floating point. Integer values are best used to represent categorical (discrete) data, and floating-point values to represent continuous surfaces.
Geographic information system
http://docs.qgis.org/2.0/de/docs/gentle_gis_introduction/raster_data.html (1.); http://webhelp.esri.com/arcgisdesktop/9.2/index.cfm?TopicName=What_is_raster_data%3F (2.)
Author: Laura Saupe