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Values

(princ (concatenate 'string "hello, " "world"))
;; => hello, world
;;
;;    "hello, world"

concatenate can be used with different types, we need to specify the output type with the 'string symbol.

(princ (format nil "1 + 1 = ~a" (+ 1 1)))
;; => 1 + 1 = 2
;;
;;    "1 + 1 = 2"

format is a function for creating formatted strings
nil tells format to evaluate to the string it creates
~a is a format directive. It prints the next argument to format _a_esthetically.
All format directives start with ~

(format t "~a" (/ 4.0 5.0))
;; => 0.8
;;
;;    NIL

floating point numbers have a . in them
when the second argument to format is t it prints the string to standard out
in this case format evaluates to nil

(+ 1/2 1/4)
;; => 3/4

Lisp has rational numbers as a separate type - they are written with a / between the numerator and the denomenator.

(and t nil)
;; => NIL

(and "ada" 5 "alonzo")
;; => "alonzo"

(and)
;; => T

(or t nil)
;; => T

(or nil "lucy" nil)
;; => "barry"

(or)
;; => NIL

(not nil)
;; => T

(not "amy")
;; => NIL

There is no true or false - only t and nil
t and nil are special symbols; they always evaluate to themselves
anything that's not nil is treated as true!
and and or can take multiple values
and returns the first nil or the last non-nil argument
or returns the first non-nil argument
not will return nil for all non-nil values, and t for nil

See also

Common Lisp Hyperspec: FORMAT Common Lisp Hyperspec: Formatted output Practical Common Lisp: A Few Format Recipes