The word crash in a computer sense doesn't necessarily mean that your computer dies or the hard drive fails. It means that you may see the BSoD (Blue Screen of Death), rebooting for no reason, shutting down for no reason, or the spinning Apple.
Heat is probably the most common reason that computers crash. As you probably know, there are fans inside your computer that help keep the computer components cool. Laptops often will overheat due to the fact that we block the airflow easier. You should try a cooling laptop desk or a table when you are working on your laptop to ensure that it will not overheat. Desktop computers can have their fans plugged easier due to the dust and other items. It's always a good idea to unplug your computer and open it to clean out the dust bunnies and spray your fan with some Dust Off.
RAM (random access memory) is a big culprit when coming to crashes. Failing RAM is not something that will happen overnight, but it may take awhile until you see the affect on your computer. If programs are crashing every time you load them, I would start to wonder if your RAM is going bad. You can easily purchase RAM at a store like Best Buy and put it in yourself (it is so easy!).
Your hard drive can begin to fail too. You will first start to see messages that say bad sector, couldn't load properly, etc. Disk access may become slower with time as well - meaning that it will take your computer longer to load or bring a program up.
I can't say it enough -- BACK UP! You never know when your computer may crash, it's an electronic system! Without a proper back up it is really hard to know if your family pictures and videos will be able to be recovered. It will also cost you a lot to have someone attempt to get that precious information off of your disk.