In which case it's time for a metaphor: The hard disk (well, disks generally, and in the old days things like tapes and punched cards) works like a filing cabinet. It's where the computer stores stuff[1]. In contrast the memory (RAM) is like the cards you have out on your desk. It's right there in a place where you can read it, but if you don't put it away in a filing cabinet by the end of the day (ie. when the computer's switched off) the cleaners will bin it and you'll lost its contents forever.
So both are important. More memory (a bigger desk to spread more cards out on) means the computer can do more stuff at once, without having to keep going back to the filing cabinet to put things away to make space all the time, which speeds things up. More disk (more filing cabinets) simply means you can store more stuff.
Because we're living in The Future, technology has progressed to the point where you can use a kind of RAM chip (one that's quite slow compared to the ones actually used for memory in proper computers, but that keeps its contents when you turn the power off) to store things instead of disks and tapes coated in magnetic stuff moving past read/write heads[1]. Wire them up in a way that pretends to work the same way as a hard drive, and put them in a case that fits the same physical space, and you've got a Solid State Disk (SSD). It's faster than a traditional hard disk, uses less power, and doesn't break if you drop it, but it's also more expensive for the same storage capacity. These are obviously ideal for laptops, where power consumption and being dropped are major considerations.
[1] Including the Operating System[3], your applications software, all your documents and media, and a load of random files the OS and applications programs have to create for their own purposes that you don't normally care about.
[2] Same sort of thing that's used to make the internal memory on your phone, SD cards, USB flash drives and so on.
[3] A suite of software that works as a middle layer between the physical computer and the useful programs you want to run. It takes high-level instructions like "draw me a window on this bit of the screen and put this text in it" or "send this packet to such-and-such-an-address-on-the-network" and turns them into a binary instructions for the graphics chips or wifi interface, as well as taking care of boring but important things like running more than one program at the same time and dealing with whereabouts on the disk a given file is actually kept. End users don't need to care about most of this stuff though, what ultimately matters is what applications can run, what type of computer you can use it on, and what the user interface (things like windows and mouses and command lines and icons and touch screens and so on work) is like. Operating systems are things like Windows, OSX, Linux, iOS and Android. Applications software is often available for multiple operating systems (eg. there's a version of the Chrome web browser for all of those), but sometimes not (eg. Microsoft Word is only available for Windows and OSX, and the OSX version's arguably bit rubbish).
