Strings are defined as an array of characters. The difference between a character array and a string is the string is terminated with a special character ‘\0’.

In C, a string is an array of characters that is terminated by a null character ('\0'). It is a sequence of characters that represent a text or a message, and is often used for storing and manipulating textual data. Strings are important data types in programming, and are used in many applications such as text processing, data storage, and communication protocols.

To declare a string in C, you need to use an array of characters. For example, you can declare a string named "message" with a maximum length of 50 characters as follows: 

char message[50];

Once you have declared a string, you can initialize it with a sequence of characters as follows: 

char message[50] = "Hello, world!";

In this example, the string "Hello, world!" is copied into the message array. Note that the null character is automatically added at the end of the string.

You can access individual characters in a string using their index. For example, to access the first character of the "message" string, you can use the following expression:

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char first_char = message[0];

You can also use various string manipulation functions in C, such as strlen(), strcpy(), strcat(), and strcmp(), to perform operations on strings. These functions are included in the string.h library in C.