a minor versus a major page fault
Not all page faults cost the same. The cheap kind, a minor (soft) fault, is fixed without touching the disk; the expensive kind, a major (hard) fault, has to read from disk before the program can continue. The difference between them is roughly the difference between finding a book already on your desk versus waiting for it to be shipped from a distant warehouse — same request, wildly different wait.
A minor fault happens when the needed data is already in RAM or needs no disk read — the OS just has to fix up a page-table entry. Examples: the page is a fresh zero page the OS can hand over instantly; the page is already resident because another process loaded it and you only need a mapping to it; or it is a copy-on-write page that just needs one in-memory copy. The cost is microseconds. A major fault happens when the page's data genuinely has to be fetched from disk — read in from the executable or a memory-mapped file the first time, or paged back from swap. The cost is dominated by the disk and can be hundreds of thousands of times slower, milliseconds rather than microseconds.
This distinction is the single most useful lens for performance. Tools that report a process's minor and major fault counts (for instance the 'min flt' and 'maj flt' columns, or /usr/bin/time -v) tell you whether a slow program is just mapping memory cheaply (minor) or being throttled by disk traffic (major). A workload whose major-fault rate keeps climbing is on the road to thrashing; one with only minor faults is paying the small, normal price of demand paging.
First touch of freshly malloc'd memory is usually a minor fault — the OS hands you a zero page already in RAM. First touch of a page that was swapped out is a major fault — the OS must read it back from the swap device before your instruction can finish.
No disk read: minor. Disk read required: major.
Both are real page faults; the split is only about whether disk I/O is needed. Major faults are the ones that hurt, and a rising major-fault rate is an early warning of memory pressure and impending thrashing, whereas plentiful minor faults are normal and cheap.