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MATHEMATICA GUIDE
Expression Structure
A foundational idea in Mathematica is that all expressions—whatever they may represent—ultimately have a uniform tree-like structure.
Reference
FullForm — the full functional form of an expression, without shortened syntax
TreeForm — a representation of the tree structure of small expressions
Head — the head of an expression: the f in 
Length — the number of elements in an expression
Part (..[[..]]) — numbered part of an expression
Level — parts specified by a given number of indices
Depth — the maximum number of indices to specify any part
Atomic Components of Expressions »
Symbol ▪ Integer ▪ Real ▪ String ▪ ...
