Userland refers to an application space, typically in unix systems which are external to the kernel. Sometimes userland can mean an application which performs its own system calls or Input/Output (IO), but typically the userland part of an application will request system calls and other system activities to be done by the kernel.
In the filesystem hierarchical sense, userland means any storage space on the system disk which is not a part of the critical system storage, ie. it is meant for users' space storage such as their personal documents and other non critical data. This can typically reside in /home or /usr/home on a unix system.