1. A method of extracting data of interest from at least one web site of a plurality of web sites, wherein the data of interest is information associated with a product, the method comprising: (A) for each respective web site W in said plurality of web sites, (i) creating a respective description of data of interest that identifies the web site W; (ii) developing an extraction pattern from a web page output from the respective web site W using a graphical user interface tool, the extraction pattern being adapted to identify at least a portion of an output of a web site and to extract information from a plurality of web pages of the respective web site W, wherein the extraction pattern comprises a regular expression; and (iii) associating the developed extraction pattern with the respective description of data of interest for the respective web site W; (B) receiving a value for use as an extraction parameter for the developed extraction patterns; and (C) obtaining said data of interest by querying the at least one web site of the plurality of web sites using the value and the extraction patterns associated with the respective descriptions of data of interest; and (D) extracting said data of interest from the at least one web site of the plurality of web sites and storing said extracted data of interest.