ADSL (Asymmetric Digital Subscriber Line) is a type of broadband communication technology that transmits digital data over standard copper telephone lines. It is termed 'asymmetric' because it provides higher download speeds compared to upload speeds, making it suitable for most home and small business internet users who typically download more data than they upload. ADSL enables simultaneous voice and data transmission, allowing users to use the internet and make phone calls at the same time. Its security implications include potential vulnerabilities if not properly secured, as data travels over public telephone infrastructure.