Microsoft Defender for Endpoint
Also known as MDE
Microsoft's enterprise EDR product, providing behavioral detection, investigation, and remote response on managed devices.
Plan 1 covers prevention: next-generation antivirus, attack surface reduction rules, and basic response. Plan 2 adds the parts that matter during an incident, including advanced hunting across telemetry, automated investigation, and six months of retained data.
Business Premium includes Defender for Business, which sits between the two and is a reasonable fit for smaller estates without the full Plan 2 investigation depth.
Where this comes up
Microsoft Defender for Endpoint sits inside our microsoft 365 and cloud work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.