Manipulating Expressions like Lists
You can use most of the list operations discussed in "Lists" on any kind of Wolfram Language expression. By using these operations, you can manipulate the structure of expressions in many ways.
You should remember that all functions which manipulate the structure of expressions act on the internal forms of these expressions. You can see these forms using FullForm[expr]. They may not be what you would expect from the printed versions of the expressions.
There are a few extra functions that can be used with expressions, as discussed in "Structural Operations".