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I am a big fan of using analogies. Sometimes they work well, offering a 1:1 relationship with the idea or concept that you are trying to explain; other times not so much. I thought I would share an analogy that I use when explaining the difference between RAM and disk drives - the kitchen.
So, if you want to produce a meal in your home, you usually have to go to the kitchen. You store food in places like the pantry, cupboards and refrigerator. These are the equivalent of your disk drive(s); instead of storing food, disk drive(s) store data. You can't work with the data when it is on your disk drive(s) just like you can't prepare a meal when the ingredients are in the pantry, cupboards and/or refrigerator. You have to take the ingredients out of these places to prepare your meal. Likewise, you have to take (load) your data from your disk drive(s) to another location to actually work with it.
That location is a thing called RAM. This is the equivalent of your counter tops in your kitchen. Just as you would mix, grind, spread or do something else with your ingredients on your counter top (with a bowl or plate of course depending on the situation), RAM is where you make edits to your data, whether that happens to be a text document, video, photo or something else. You could probably throw in your stove, oven and microwave here too, depending on what exactly you are doing to your data.
Once you are finished making edits to your data you save the new version of whatever it is back to your disk drive. This is like your leftovers from your meal. You normally wouldn't leave it out on the counter top (unless maybe you're a college student who had pizza that evening :P); rather, you would put it back into storage, most likely the refrigerator.
So, in a nutshell, this is why you need disk drives and RAM and what their roles are in your computer.