Looking on Wikipedia (Cymbalta; Celexa), the first difference I notice is that Cymbalta is an SNRI and Celexa is an SSRI. So the difference is the Celexa does not inhibit the reuptake of norepinephrine. Depending on whether your particular brain chemistry has an adequate amount of norepinephrine, you may or may not notice a difference in effectiveness. You can look at the linked pages to compare the side effects, which seem to be mostly similar.