How does an SSD work
Even though they're often referred to as disks, Solid State Drives (SSDs) have no spinning platter like a traditional hard drive. Instead they act more like the RAM inside your computer (except they retain the information when the power is off). This is called NAND Based Memory (NAND isn't an acronym incidentally, but a term used in logic).
Data is written to electrical blocks that are set to on and off (for 0s and 1s). There's no magnetic or electro mechanical part. Modern drives also integrate DRAM with the Flash memory to further improve performance, and to cache data in case of a power failure.
What's good about SSD
Speed is the number one factor to consider when picking an SSD drive. In testing a typically hard drive will return somewhere between 50-100Mbps depending on the speed and quality of the drive. With an SSD you can expect that to rise to somewhere between 300MBps and 500Mbps. So it's roughly going to be between 5x to 10x faster at reading data. This makes a huge difference when running your Mac: opening documents, starting programs and even booting up all happen much faster. You don't have to watch icons bouncing in the dock, and you'll see far less of the spinning rainbow 'waiting' icon. Going back to a regular hard drive after used an SSD for a while is extremely difficult.
Because SSD drives do not have a spinning platter they are also completely silent in operation. The only sound you'll hear from your Mac is the sound of the fan in use, which is often also very quiet in most Macs. No more clicking and whirring.
Is a HDD more reliable than an SSD
SSD drives are assumed to be more reliable than HDD drives. Because they do not have a spinning platter, or motor, or moving actuator arm it is generally thought that there's less moving parts that are likely to go wrong. Of course this is weighted against the fact that HDD development is many years in, and SSD technology is being rapidly improved. Many SSD drives have shown errors or problems, Crucial had trouble with memory leaks in its M4 model (although this was fixed with a Firmware update.) There's little reliable information about long-term reliability of SSDs, but the general assumption is that they'll last longer.
What is a Fusion Drive
A Fusion Drive is Apple's branded hybrid drive that mixes the best of SSD with HDD technology. Inside the Fusion Drive is a SSD NAND memory storage block, and a HDD platter. Where it gets clever is the integration with the Mac OS X operating system. Mac OS X quickly figures out what files, applications and data blocks you use on a regular basis and moves these to the speedy SSD, meanwhile the big files that you might only access a few times (say a movie file) is moved to the HDD. So the Mac mostly runs as quickly as a pure SSD drive expect for when you need those big files.
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