Append[expr, elem] gives expr with elem appended.
Examples: Append[a,b, c] ; Append[f[a], b+c] .
In iteratively building a structure, it is usually more efficient to use list, new at each step, followed by Flatten at the end, than to use Append[list, new] at each step.
See The Mathematica Book: Section 1.8.6.
See also: Prepend, Insert, AppendTo, PadRight.