You don't specify anything, so this answer is extremely generic:
The POSIX standard specifies many different communications methods. While most are Inter-process communication techniques (IPCs) , there are some IPC methods that include kernel communication, e.g.: Signals and Timers.
However, the most common method for a userland application to contact the kernel is via a syscall.
http://en.wikipedia.org/wiki/System_call
If you ask a more specific question, you'll get a more specific answer.