Windows provides a set of common utility routines needed by various kernel-mode components. For example, RtlCheckRegistryKey is used to see if a given key is in the registry.
Most of the run-time library (RTL) routines are prefixed with the letters "Rtl"; for a list of the run-time library routines for the kernel, see Run-Time Library (RTL) Routines.
There is also a different kernel-mode library specifically designed for safe string handling. For more information about the safe string library, see Windows Kernel-Mode Safe String Library. Note that safe string library routines are also usually prefixed by "Rtl" but are not part of the run-time library (RTL).