EMZETT.SSDs
In short: “Solid State Drives” — mass storage with no moving parts, storing data in flash memory chips. Considerably faster than HDDs, especially for random access.
In more detail: SSDs come in various connection types: classically as a 2.5-inch drive via SATA, or as considerably faster M.2 modules via NVMe/PCIe. Without mechanical parts, SSDs are also more resistant to shocks and quieter than HDDs.
In Depth
SSDs store data in NAND flash memory cells — tiny transistors that store electrical charge permanently (even without power) and can represent one or more bits of information depending on their charge state. Without rotating platters or moving read heads like HDDs, the mechanical seek time is almost entirely eliminated, which is why SSDs are many times faster than HDDs especially for many small, random accesses (e.g. when booting an operating system with thousands of small files) — for purely sequential reading of large, contiguous files, the difference is smaller, but still considerable.
A technical peculiarity of flash memory: individual memory cells slowly wear out through repeated writing (a limited number of write cycles) — modern SSD controllers therefore actively spread write operations across all available cells (“wear levelling”), so no single cell wears out prematurely. For end users, this lifespan is barely relevant in practice, since modern SSDs last many years to decades under normal use. Regarding connection type, a distinction is made between older SSDs connected via SATA (compatible with classic HDD slots, but throttled by SATA) and modern M.2 SSDs via NVMe/PCIe, which reach considerably higher speeds.
DRAM cache and TLC/QLC memory
Many SSDs also include a small, fast DRAM cache (similar to the CPU cache principle), which caches frequently needed mapping tables and thereby noticeably improves access speed — cheaper, “DRAM-less” SSDs forgo this and can be noticeably slower under certain workloads (many small, random write accesses), even though the pure flash memory chip could be identical. With flash memory itself, a further distinction is made by how many bits are stored per memory cell: SLC (1 bit, fastest and most durable, but expensive), MLC (2 bits), TLC (3 bits, today the most common compromise) and QLC (4 bits, cheapest, but slowest and fastest-wearing variant).
The TRIM command
For an SSD to maintain its speed over the long term, the operating system has to tell it, via the TRIM command, which storage areas contain deleted files and can actually be internally freed up — without TRIM, the SSD firmware doesn’t find out in time that an area is free again, and has to perform additional, time-consuming internal erase steps on the next write. Modern operating systems automatically enable TRIM by default for detected SSDs.