In computing, a system resource, or simply resource, is any physical or virtual component of limited availability within a computer system. Every device connected to a computer system is a resource. Every internal system component is a resource. Virtual system resources include files (concretely file handles), network connections (concretely network sockets), and memory areas. Managing resources is referred to as resource management, and includes both preventing resource leaks (releasing a resource when a process has finished using it) and dealing with resource contention (when multiple processes wish to access a limited resource).
CPU, both time on a single CPU and use of multiple CPUs – see multitasking
Random access memory and virtual memory – see memory management
Hard disk drives, include space generally, contiguous free space (such as for swap space), and use of multiple physical devices ("spindles"), since using multiple devices allows parallelism
Cache space, including CPU cache and MMU cache (translation lookaside buffer)
System resource Wikipedia
Some resources, notably memory and storage space, have a notion of "location", and one can distinguish contiguous allocations from non-contiguous allocations. For example, allocating 1 GiB of memory in a single block, versus allocating it in 1,024 blocks each of size 1 MiB. The latter is known as fragmentation, and often severely impacts performance, so contiguous free space is a subcategory of the general resource of storage space.
One can also distinguish compressible resources from incompressible resources. Compressible resources, generally throughput ones such as CPU and network bandwidth, can be throttled benignly: the user will be slowed down 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. Electrical power is to some degree compressible: without power (or without sufficient voltage) an electrical device cannot run, and will stop or crash, but some devices, notably mobile phones, can allow degraded operation at reduced power consumption, or can allow the device to be suspended but not terminated, with much lower power consumption.