If you are looking for a way to scrape data off of websites that do not have an API, here is a simple how-to video for it.
Data scraping is the process of extracting data from output that was originally intended for humans. A web page is an example of output originally intended for humans in contrast to an API intended for use by other programs.
Nokogiri is a Ruby Gem that extracts data from web pages using CSS selectors. Additionally, it provides methods to help parse (make sense of) the results. The use of CSS selectors allows you to easily target the data you wish to extract from a URL.