Web contains an overwhelming amount of useful data that might be difficult or expensive to get from any other source. While much of the web data is unstructured in nature, there is also a significant amount of embedded structured data, such as product information on e-commerce sites, real estate listings or stock data on financial sites. This data is typically a description of objects retrieved from underlying relational databases and displayed in Web pages following some fixed templates. Web data extraction software is a tool that allows easy and robust extraction of structured data from template based HTML sources and conversion of such data back into a relational format that can be loaded into a spreadsheet or a database. Web data extraction logic is similar to the logic used by search engines when they index Web content. The difference is that web data extraction software focuses more on the translation of semi-structured dynamic Web content int

o relational data suitable for analysis and offline access.

Legal web data extraction should not mixed with scraper sites that are usually operated by spammers. They copy most of the content of the scraper site from other websites. By doing so, they try to exploit the popularity of the material from original content providers to attract search engine traffic to their sites to make money through advertising.

Writing data scraping code for a particular site is not a terribly hard technical problem, but it is also far from trivial given all the weird things that can happen on websites these days. In many applications, such as comparison shopping, data has to be extracted from many different sources, making manual coding of a data scraper for each source impractical. On the other hand, fully automatic approaches are often not reliable enough, resulting in low quality of the extracted data. The most practical solution is a semi-automatic data extraction when a user of a web scraper defines a typical data pattern and lets the program to crawl through the web pages collecting matching content. The main goal of the developers of web scraping software is to achieve reliable extraction with a minimal amount of user effort.

Web data extraction software is a powerful tool to import web data into Microsoft Excel spreadsheet for analysis and research. Excel has a built in data extraction system called Web Query. The main weakness of Web Query is that it relies on HTML table elements inside the web page to specify the content to extract. Presently, table-based web page design has been almost completely replaced by Cascading Style Sheets (CSS). Web Queries do not recognize data patterns presented using CSS. Web scraping software enhances Web Query engine by providing an automated tool that supports flexible CSS based content, client-side scripting, password protected web sites and complex navigation rules. It converts HTML to CSV file or other format compatible with Excel.

One of the top suppliers of a web data extraction software, DataTool Services, is a company that provides users with the ability to affordably and simply collect and reuse web content. The adaptable algorithm implemented in the company's popular product Data Toolbar allows data extraction from all kinds of web pages including dynamic AJAX, login protected pages, complicated HTML pages with nested data lists, and much more. This software can export information to a variety of formats supported by Excel and database engines.