SLIME uses a custom Read-Eval-Print Loop (REPL). The REPL user-interface is written in Emacs Lisp, which gives more Emacs-integration than the traditional comint-based Lisp interaction:
Load the REPL:
(add-to-list 'slime-contribs 'slime-repl)
Select the output buffer, preferably in a different window.
M-x slime-switch-to-output-buffer (C-c C-z)
Insert a call to the function defined around point into the REPL.
M-x slime-call-defun (C-c C-y)
Inserts the last expression to the REPL and evaluates it there.
M-x slime-eval-last-expression-in-repl (C-c C-j)