Split[list] splits list into sublists consisting of runs of identical elements.
Split[list, test] treats pairs of adjacent elements as identical whenever applying the function test to them yields True.
Example: Split[a, a, b, b, a, a, b] .
The default function used to test whether elements are identical is SameQ.
Split can be used to perform run-length encoding.
See The Mathematica Book: Section 1.8.10.
See also: Partition, Union, Flatten, ReplaceList.