Windows 10 telemetry?

Windows 10 telemetry data includes basic system diagnostics information, logs of how frequently you use features and applications, system files, and likely more metrics that have yet to be disclosed. Fortunately, Microsoft allows users to set the amount of data they wish to share, but it cannot be completely turned off. Enterprise installations have an even more minimal setting but even then a small amount of security and diagnostic information is sent.

Microsoft splits telemetry settings into four tiers.

Security – Data and logs pertaining to Windows security, including client settings and metrics collected from the Malicious Software Removal Tool and Windows Defender. Available only on Enterprise installations.
Basic – Device information including application compatibility and security.
Enhanced – Everything from the Security and Basic tiers, plus data about how Windows and Windows application use and performance.
Full – Everything from the three other tiers and more. This information can include user content that may have triggered a crash or problem.

Windows 10 transmits the following data back to the company:

Typed text on keyboard sent every 30 minutes
Transmits anything you say into a microphone
Transcripts of things you say while using Cortana
Index of all media files on your computer
When your webcam is first enabled, 35mb of data
Telemetry data

What is Telemetry?

Telemetry, in general, is a term for technologies that accommodate automatic recording and collecting information in the form of measurements or statistical data, and forward it to IT systems in a remote location for monitoring and analysis. This term can be used in reference to many different types of systems, such as wireless systems using radio, ultrasonic or infrared technologies, or some types of systems operating over telephone or computer networks. Others may use different strategies like SMS messaging.