LAS is a file format specification for the interchange of 3D point data, which is maintained by committees within the American Society for Photogrammetry and Remote Sensing (ASPRS) as directed by its board.
The LAS format stores an x, y, z coordinate per point. In addition, information such as the actual intensity, or magnitude, of the return value and user defined values, such as point classification can be stored per point. The return value, or return number, is the pulse return number for a given output pulse. A given output laser pulse can have many returns, and they are marked in sequence of return. The first return may have a return number of one, the second a return number of two, and so on up to five returns. The file format consists of the file extension *.las.
Though the LAS format is widely adopted and used throughout the industry, it is not spatially indexed and it does not have a provision for generalizations, both of which are problems when trying to work with very large data sets. Spatial indexing provides a means of locating all the points within a given area quickly without scanning the whole file and generalization allows for a representative subset of the points to be used for visualization at small scales.
LAS Specification document is available at ASPRS website > asprs.org > Standards section.