DEFPARAMETER always assigns a value. So:
[1]> (defparameter a 1)
A
[2]> (defparameter a 2)
A
[3]> a
2
while DEFVAR does it only once, so:
[4]> (defvar b 1)
B
[5]> (defvar b 2)
B
[6]> b
1
SETF is a macro which uses SETQ internally, but has more possibilities. In a way it’s a more general assignment operator. E.g. with SETF you can do:
[19]> (defparameter c (list 1 2 3))
[21]> (setf (car c) 42)
42
[22]> c
(42 2 3)
but you can’t do that with SETQ:
[23]> (setq (car c) 42)
*** - SETQ: (CAR C) is not a symbol
The following restarts are available:
USE-VALUE :R1 Input a value to be used instead.
ABORT :R2 Abort main loop
Break 1 [24]> abort