What is the difference between a regular string and a verbatim string?

A verbatim string is one that does not need to be escaped, like a filename:

string myFileName = "C:\\myfolder\\myfile.txt";

would be

string myFileName = @"C:\myfolder\myfile.txt";

The @ symbol means to read that string literally, and don't interpret control characters otherwise.

This is covered in section 2.4.4.5 of the C# specification:

2.4.4.5 String literals

C# supports two forms of string literals: regular string literals and verbatim string literals.

A regular string literal consists of zero or more characters enclosed in double quotes, as in "hello", and may include both simple escape sequences (such as \t for the tab character) and hexadecimal and Unicode escape sequences.

A verbatim string literal consists of an @ character followed by a double-quote character, zero or more characters, and a closing double-quote character. A simple example is @"hello". In a verbatim string literal, the characters between the delimiters are interpreted verbatim, the only exception being a quote-escape-sequence. In particular, simple escape sequences and hexadecimal and Unicode escape sequences are not processed in verbatim string literals. A verbatim string literal may span multiple lines.

In other words the only special character in a @"verbatim string literal" is the double-quote character. If you wish to write a verbatim string containing a double-quote you must write two double-quotes. All other characters are interpreted literally.

You can even have literal new lines in a verbatim string literal. In a regular string literal you cannot have literal new lines. Instead you must use for example "\n".

Verbatim strings literals are often useful for embedding filenames and regular expressions in the source code, because backslashes in these types of strings are common and would need to be escaped if a regular string literal were used.

There is no difference at runtime between strings created from regular string literals and strings created from a verbatim string literals - they are both of type System.String.