Here is a discussion of string content vs buffer content in the Emacs wiki. Just store your string as a variable.
The tricky thing about strings is that you generally do not modify the string itself (except if you perform array functions on string, since a string is an array, but this should generally be avoided), but you return the modified string.
At any rate, here is an example of using a string in elisp.
This will trim the whitespace from the end of a string:
(setq test-str "abcdefg ")
(when (string-match "[ ]*$" test-str)
(message (concat "[" (replace-match "" nil nil test-str) "]")))
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