As a user or developer of MnemOS, you are likely to run into two main parts, the kernel and userspace.
The kernel handles hardware-level operations, including memory management, event handling for hardware and driver events, and isolation of userspace from the hardware.
The userspace is where applications run. Applications are provided a standard interface from the kernel, that allows them to perform operations like reading or writing to a serial port, or reading or writing to a block storage device (sort of like a hard drive).
Additionally, there is a common library, which contains software components used by both the kernel and userspace.