Web scraping is a process of retrieving desired data from websites. It is used when a site from which you want to extract some data does not have an API, or web scraping is the only option.
HTML scraping and data crawling
P.S.: For your information, if a target site noticeably changes its layout, a website parser code attached to it probably will be broken. It can be fixed fast if a change is moderate or should be almost completely rewritten otherwise, if a paradigm of data retrieving is changed.