In an UNIX-like system, a system call (syscall) is used to request all kinds of functionality from the operating system kernel.

A microkernel-based system typically won't offer a lot of system calls -- apart from one central one, and that is send message -- but instead RPCs will be used instead. See GNU Mach's system calls.

In the GNU Hurd, a lot of what is traditionlly considered to be a UNIX system call is implemented (primarily by means of RPC) inside glibc.