The Linux kernel often abstracts its interfaces. An interface is a collection of routines and data structures which operate in a well-defined way. For example all, network device drivers have to provide certain routines to operate on particular data structures. This way there can be generic layers of code using the services (interfaces) of lower layers of specific code. The network layer is generic and it is supported by device specific code that conforms to a standard interface.

Often these lower layers register themselves with the upper layer at boot time. This registration usually involves adding a data structure to a linked list. For example each filesystem built into the kernel registers itself with the kernel at boot time or, if you are using modules, when the filesystem is first used. You can see which filesystems have registered themselves by looking at the file /proc/filesystems.

The registration data structure often includes pointers to functions. These are the addresses of software functions that perform particular tasks. Again, using filesystem registration as an example, the data structure that each filesystem passes to the Linux kernel as it registers includes the address of a filesystem specfic routine which must be called whenever that filesystem is mounted.