What is the difference between virtual memory (VIRT) and resident memory (RES) in top, and what is shared memory (SHR)?

VIRT is the process's virtual address space, while RES is the portion currently resident in physical memory.

SHR describes resident memory that may be shared, not necessarily memory uniquely attributable to another process. Summing RES across processes can double-count shared pages; VIRT alone does not indicate physical RAM consumption.