By default babel will only allow you to execute emacs-lisp source code blocks. You can enable babel to allow execution of code blocks in a bunch of different languages though — you can find a full list in the Org Mode Manual.
(org-babel-do-load-languages 'org-babel-load-languages '((emacs-lisp . t) (python . t) (ruby . t)))
To evaluate a block, type C-c C-c.