Simplistic answer:
Unlike Micro$oft's 'OS'es, unix and linux still have easily distinguishable distinctions between the kernel and applications that run on top of it. The kernel is the OS proper - it has complete access to the hardware, and completely unfettered control over things like interupts, virtual memory management, CPU schedling etc, so buggy kernel code can crash the system (ie empeg) hard. 'Userland' is basically any other non-kernel code - buggy code here is likely to crash that particular app, but the system should still deal with it gracefully.