What is demand paging and what is thrashing?

Demand paging loads pages into memory only when accessed (lazy loading), not at process start. On page fault: if page is in swap, load it; else it's a new access, allocate frame. Benefits: faster startup, less memory usage, run programs larger than RAM. Thrashing occurs when system spends more time handling page faults than executing - too many processes compete for limited frames. Solutions: working set model, page fault frequency control, swapping out processes.