, , ... is a list of elements.
Lists are very general objects that represent collections of expressions.
Functions with attribute Listable are automatically "threaded" over lists, so that they act separately on each list element. Most built-in mathematical functions are Listable.
a, b, c represents a vector.
a, b, c, d represents a matrix.
Nested lists can be used to represent tensors.
See The Mathematica Book: Section 1.2.3 and Section 1.8.
See also: Sequence.