elisp: Printed Representation
2.1 Printed Representation and Read Syntax
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The “printed representation” of an object is the format of the output
generated by the Lisp printer (the function ‘prin1’) for that object.
Every data type has a unique printed representation. The “read syntax”
of an object is the format of the input accepted by the Lisp reader (the
function ‘read’) for that object. This is not necessarily unique; many
kinds of object have more than one syntax. 
Read and Print.
In most cases, an object’s printed representation is also a read
syntax for the object. However, some types have no read syntax, since
it does not make sense to enter objects of these types as constants in a
Lisp program. These objects are printed in “hash notation”, which
consists of the characters ‘#<’, a descriptive string (typically the
type name followed by the name of the object), and a closing ‘>’. For
example:
(current-buffer)
⇒ #<buffer objects.texi>
Hash notation cannot be read at all, so the Lisp reader signals the
error ‘invalid-read-syntax’ whenever it encounters ‘#<’.
In other languages, an expression is text; it has no other form. In
Lisp, an expression is primarily a Lisp object and only secondarily the
text that is the object’s read syntax. Often there is no need to
emphasize this distinction, but you must keep it in the back of your
mind, or you will occasionally be very confused.
When you evaluate an expression interactively, the Lisp interpreter
first reads the textual representation of it, producing a Lisp object,
and then evaluates that object (Evaluation). However,
evaluation and reading are separate activities. Reading returns the
Lisp object represented by the text that is read; the object may or may
not be evaluated later. Input Functions, for a description of
‘read’, the basic function for reading objects.
Info Catalog
elisp: Lisp Data Types
elisp: Comments