This presentation defines APIs, shows how to use them and how to circumvent practical problem in their invocation, through crawling strategies and scraping.
A crawler gets web pages or web API invocations by applying complex strategies, i.e. to download whatever is linked to from the starting page on the web, or all the data you want from the API.
A scraper takes web pages that have been downloaded and (attempts to) extract data from those pages, so that it can be stored and analysed.
Note that scraping may violate the rights of the owner of the information and/or user agreements about use of web sites.