Screen-scraping, also known as web-scraping or data-scraping is a software technique used to collect and parse information from web sites. The information is scraped via a parser, for example using regular expressions or, in the case of a 3270 emulator, variants of HLLAPI.
Questions that should have this tag would be directly related to gathering information from websites through the use of a parsing mechanism such as regular expressions. (Questions about screen-scraping using regular expressions should also be tagged regex.)
Because information on web pages is almost certainly organized in well-formatted HTML, basic screen-scraping can be a simple task. In most cases, the reason for screen-scraping is to not only parse the data on the web page, but then to collect it either by reproducing it on a different web page or storing in a database.
Historically, screen-scraping also described the technique of "scraping" data off of or on to a 3270 emulator. This technique gained some popularity shortly after the advent of such emulators. The API 3270 emulators implemented was known as HLLAPI (High Level Language Application Programming Interface), later EHLLAPI (Enhanced HLLAPI) and WinHLLAPI came into existence. Application programs would "drive" the emulator, sending simulated keystrokes and function keys, then waiting for responses.