Solid State Dirves are semiconductor types of storage device. At their core, they use non-volatile flash memory, similar to what we can find in USB drives or SD cards used in cameras and phones. Unlike in HDDs (Hard Disk Drives), you won't find any mechanical elements inside an SSD. Only billions of transistors tightly packed on a circuit board. It's exactly those semiconductor type elements, called memory cells, that store our data (pictures, music, documents) in the form of zeros and ones. I'm oversimplifying as much as possible here, but the only other thing we need for all of it to work is a special controller which gives out "addresses" of the cells that need to be overwritten (or read). And, what's most important, the operation happens in many cells at once.
In Hard Disk Drives reading and writing data works completely differently. Firstly, all data is stored on magnetic platters. The disk controller needs to turn the platter, position the head, and only then can it start reading the data. If what we need has been written sequentially (one after another), the whole process should go more or less smoothly. But if the disk is badly fragmented, the situation becomes more complicated, as the head has to be positioned time after time, while it tries to read our data scattered around the platter. As a result, just searching the disk means additional 5-20 ms when using an HDD.
SSD is faster and safer
SSD works 100 times faster. Of course that doesn't mean that in practice they're a 100 times faster than HDDs, but it's this parameter that translates into the OS booting up significantly faster, better speeds when loading apps, and levels in video games, or when copying files. The biggest difference can be spotted when installing large pieces of software, or working in apps that have to often read from heavy files stored on disk (e.g. video editing.)
No mechanical parts in SSDs means they are insusceptible to things like bumps, or falls - very common when moving the laptop. SSDs are also more energy-efficient, lighter, and... quieter, which makes them a decidedly better option for laptops.
SDD is not the end of it: SATA III
For many years now disks in our laptops have been jammed into 2.5" cases. Both HDDs and SSDs come in this standard, and one more thing they have in common is the Serial ATA III interface (SATA), which debuted in 2009. Theoretically it offers transferring speed of 6 Gb/s, but in practice it's more or less 4.8 Gb/s (600 MB/s.) It's more than enough for HDDs, as for their platters, which rotate at 7200 rpm, have a hard time with going any faster than 100 MB/s. However, connect an SSD to it and you'll get transfers of 530 and 500 MB/s for reading and writing data respectively. It's, of course, way more than what old HDDs are capable of, but still pales in comparison to what modern semiconductor drives have to offer.
M.2 – It's just a connector!
In order to substitute the 10yo SATA III interface with something more modern, hardware producers came up with the M.2 connector. It's been designed to be a standardized slot which we can use to connect not only high-end SSDs, but also expansion cards equipped with other interfaces like Bluetooth, NFC, GPS, or Wi-Fi.
The most important thing is that the M.2 allows us to bypass the bottleneck that is the SATA III. It's possible thanks to the fact that it can (but doesn't have to!) use the ultrafast PCI-Express (PCIe) interface to communicate with the motherboard. SSDs compatible with PCI-E 3.0x4 can achieve transfer speeds of up to 3500 MB/s for writing, and 2000 MB/s for reading data. That's seven times better than if we used an SSD with SATA!
It's important to keep in mind that M.2 is just a connector. SSDs compatible with it can come in both SATA and PCIe variations. And to make things more difficult, the latter can use one of two types of protocols to communicate with the OS: the archaic, all but abandoned AHCI, which was never meant to work with PCIe connected SSDs, and the modern, fast, and energy-efficient NVMe (Non-Volatile Memory Express.) We're interested in that last one.
What drive for my notebook should I choose
In 2020, SSDs should be considered a standard in the laptop market. As we stated before they're faster and tougher than classic HDDs. However, what type of an SSD are we going to find in our notebook exactly comes down to how much is it going to cost. The optimal configuration would be a 256-512 GB PCIe SSD as a main drive, and a second, bigger PCIe or SATA III SSD for storing data.