Head
Head[expr]
returns the head of the expression.
Attributes[Head] := {Protected}
Simple examples
In[1]:= Head[f[x]] Out[1]= f
In[2]:= Head[x] Out[2]= Symbol
In[3]:= Head[{x}] Out[3]= List
In[4]:= Head[a + b] Out[4]= Plus
In[5]:= Head[1] Out[5]= Integer
In[6]:= Head[1.] Out[6]= Real
In[7]:= Head[(2)/(7)] Out[7]= Rational
In[8]:= Head[(1)/(7)] Out[8]= Rational
In[9]:= Head["1"] Out[9]= String
In[10]:= Head[Head[a + b[x]]] Out[10]= Plus