The DRM core exports several interfaces to applications, generally intended to be used through corresponding libdrm wrapper functions. In addition, drivers export device specific interfaces for use by userspace drivers & device aware applications through ioctls and sysfs files.
External interfaces include: memory mapping, context management, DMA operations, AGP management, vblank control, fence management, memory management, and output management.
Cover generic ioctls and sysfs layout here. Only need high level info, since man pages will cover the rest.