What is UTF-8?

[chinmay.sahoo]

UTF-8 (UCS Transformation Format—8-bit) is a variable-width encoding that can represent every character in the Unicode character set. It was designed forbackward compatibility with ASCII and to avoid the complications of endiannessand byte order marks in UTF-16 and UTF-32. UTF-8 has become the dominant character encoding for the World Wide Web, accounting for more than half of all Web pages. The Internet Mail Consortium (IMC) recommends that all e-mail programs be able to display and create mail using UTF-8. UTF-8 is also  increasing being used as the default character encoding in operating systems,programming languages, APIs, and software applications

[RH-Calvin]

UTF-8 is a character encoding capable of encoding all possible characters, or code points, in Unicode. The encoding is variable-length and uses 8-bit code units.

[TomClarke]

UTF-8 is a method for encoding Unicode characters using 8-bit sequences.
Unicode is a standard for representing a great variety of characters from many languages.
UTF-8 is often used for encoding  XML/XHTML files, but you can encode other text formats in UTF-8 too if the libraries used to read the file can deal with this encoding.