All connected devices and internal system components are resources.
Virtual system resources include files (concretely file handles), network connections (concretely network sockets), and memory areas.
[1] Compressible resources, generally throughput ones such as CPU and network bandwidth, can be throttled benignly: the user will be slowed proportionally to the throttling, but will otherwise proceed normally.
Other resources, generally storage ones such as memory, cannot be throttled without either causing failure (if a process cannot allocate enough memory, it typically cannot run) or severe performance degradation, such as due to thrashing (if a working set does not fit into memory and requires frequent paging, progress will slow significantly).
The distinction is not always sharp; as mentioned, a paging system can allow main memory (primary storage) to be compressed (by paging to hard drive (secondary storage)), and some systems allow discardable memory for caches, which is compressible without disastrous performance impact.