LeafCount[expr] gives the total number of indivisible subexpressions in expr.
LeafCount gives a measure of the total "size" of an expression.
LeafCount counts the number of subexpressions in expr which correspond to "leaves" on the expression tree.
Example: LeafCount[1 + a + b^2] .
LeafCount is based on FullForm representation of expressions.
Numbers with heads Rational and Complex are treated as composite objects, just as in FullForm.
See The Mathematica Book: Section 2.13.4.
See also: ByteCount, Length, Depth, AtomQ.