Final answer:
A distorted string of characters when logging into an information system is known as a CAPTCHA, designed to differentiate between human users and automated software.
Step-by-step explanation:
When you are instructed to enter a string of characters that appear distorted onscreen, you are encountering a kind of access control known as a CAPTCHA. CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart. It is a test that is used to ensure that the response is generated by a human, rather than a computer program. This is often used to prevent bots from accessing web-based services, automated form submissions, and more. CAPTCHA typically involves distorted letters and numbers that humans can decipher but present challenges for automated software.