“When you go to a webpage, and you make a request, that parses part of the data in the request back to a server,” Hunt said. “For example, you read a news article, and the news article, in the address bar it has, “id=1”, and that gives you news article number 1, and then you get another one with ID 2.”

But, “with a SQLi attack, an attacker changes that ID in the address bar to something that forces the database to do something it’s not meant to do,” Hunt said, such as returning a piece of private data.

Source: The History of SQL Injection, the Hack That Will Never Go Away | Motherboard

Another commonly used piece of software is sqlmap. “It crawls the pages on the website, similar to how a search engine crawler might, looks for input forms on the website, and submits the forms with inputs that might cause a MySQL syntax error,” Al-Bassam added.