Feature Updates
These updates take place around March and September. Such an update also changes the four digit version number (?). The first two digits indicate the year, the last two digits the month. Thus, the feature update of March 2019 introduced the version Windows 10 1903.
Quality updates
A quality update provides security fixes and critical updates.
These updates are scheduled on the second Tuesday of each month (aka «Patch Tuesday») but can be released any time in urgent cases.
A quality update is cumulative: installing the most recent update also installs all previous updates (for a specific feature update).