The basic idea behind Topicalizer is to provide a tool with which a user can analyse texts regarding various linguistic aspects including type / token frequency, lexical density, sentence and paragraph structure, readability, word frequencies, collocations and possible keywords.
Furthermore, with Topicalizer a user can have an abstract automatically generated for a text in order to be given some insight into what that text might actually be about.

A text can be provided either as plain text or by specifying the URL where the text is to be found. If Topicalizer encounters a text that is encapsulated in valid HTML, XHTML or XML it is capable of parsing and subsequently analysing it as well.

After automatically recognising the language of the text, Topicalizer can use language-specific settings like stop words, syllable structure etc., which renders the results much more relevant.

Another interesting feature of Topicalizer is its API (http://www.topicalizer.com/api/), which can be used to embed the functionality of Topicalizer in any kind of application that has access to the Internet.

The most obvious purpose is to offer web authors a tool with which they can optimise their texts regarding complexity, readability and last but not least for search engines.
Apart from the analysis features, Topicalizer automatically suggests keywords and creates an abstract for a given text and therefore can be used for tagging texts with semantic information, too.