Documentation

Mathematica Teacher's Edition

Built-in Functions

Lists and Matrices

Structure Manipulation

Flatten

Flatten[list] flattens out nested lists.
Flatten[list, n] flattens to level n.
Flatten[list, n, h] flattens subexpressions with head h.

Example: Flatten[a,b,c,d].
Flatten "unravels" lists, effectively just deleting inner braces.
Flatten[list, n] effectively flattens the top level in list n times.
Flatten[f[e, ... ]] flattens out subexpressions with head f.
See The Mathematica Book on the web: Section 1.8.14 and Section 2.2.10.
See also: Partition.

Further Examples