QField is a mobile app for fieldwork with QGIS projects. In QGIS you prepare a map and a data-entry form, and then collect data on site with precise positioning, photos, video, audio, QR codes and NFC. The app works fully offline or connected, supports formats such as GeoPackage, KML and GPX, and can be extended with plugins written in QML and JavaScript. For synchronisation and collaboration between field and office there is the optional QFieldCloud platform.
QField is developed by OPENGIS.ch, a Swiss company with offices across Switzerland that also provides QGIS development, training and support. The software is licensed under the GPL version 2 or later, so you can inspect and modify the source code. According to its makers, QFieldCloud is available as a service on Swiss data centres or as a private cloud installation; if you use only the app, your data stays on your own devices and servers.