Wolfram Language Fast Introduction for Programmers
About the Wolfram Language »

Symbolic Expressions

Everything in the Wolfram Language is a symbolic expression.

numbers strings images arrays graphs formulas documents interfaces code ...

All symbolic expressions have the same fundamental structure: head[arguments]

X

X

X

X

The argument to a function can be any symbolic expression:

In[1]:=
X
Out[1]=

The Wolfram Language is fully symbolic, so undefined variables can always just
stand for themselves:

In[2]:=
X
Out[2]=

FullForm always shows the underlying structure.
Head always gives the head of an expression; Length gives the number of arguments.