Mac computer crashing
Macs crash when something has gone horribly wrong. When a programme 'crashes' it generally either freezes completely or unexpectedly quits. In general, when one programme crashes, it doesn't affect the other programmes that are running.
Sometimes the whole system crashes so nothing works. Typically the clock (top right of your screen) stops completely. Or the Mac just reboots.
Don't ignore crashes. They are often a sign that something serious is going on (such as the data on your hard disk has become corrupted) and if you don't check for problems (possibly by running 'First Aid' from the Disk Utility programme) and fix them, the machine will eventually stop working completely. At that point, the only option is usually to erase everything on the hard disk and start from scratch. All your data is gone at that point and you can only restore from the backup you've been making religiously every day. You have been making backups, haven't you? If not, ask us about Time Machine!