The header file string.h in the C programming language provides functions for manipulating C-style strings, which are arrays of characters terminated by a null character. It includes essential functions for tasks such as copying, concatenating, comparing, and searching strings, making it a vital tool for string handling in C programs. Common functions found in string.h include strcpy, strcat, strcmp, and strlen. These functions help developers efficiently manage string data, ensuring that operations like copying and measuring string length are performed correctly and safely, thus enhancing the overall functionality of C applications.