Internationalization is the process of changing the source code of a program so that it can meet the expectations of users in any culture, if culture specific data (translations, images etc.) are provided.
Use of Unicode is not strictly required for internationalization, but it makes internationalization much easier, because operations that need to look at specific characters (like hyphenation, spell checking, or the automatic conversion of double-quotes to opening and closing double-quote characters) don't need to consider multiple possible encodings of the text.
Use of Unicode also enables multilingualization: the ability of having text in multiple languages present in the same document or even in the same line of text.
But use of Unicode is not everything. Internationalization usually consists of three features: