Since strings are vectors which are sequences (both covered in other concepts) all functions that work on sequences will work on strings - such as position, map, remove, concatenate etc..
There are also more string specific functions to do things such as compare strings to see which is alphabetically more or less string< and string>; functions to trim characters from the left, right or both sides of a string (string-trim).
Refer to the Hyperspec Section on strings for full details.