EMZETT.M.2
In short: A compact, cardless form factor for SSDs that plug directly into the motherboard — without separate cables for data and power.
In more detail: Depending on the motherboard, M.2 slots can drive either SATA or NVMe/PCIe SSDs — the physical connector alone therefore says nothing about the actual speed, that depends on the protocol used. The compact design makes M.2 particularly attractive for laptops.
In Depth
M.2 is, first and foremost, just a physical form-factor specification — it defines how big and shaped the card is (given as a four-digit number like “2280”, where the first two digits give the width and the last two the length in millimetres), but says NOTHING about which transfer protocol underlies it. This is the main reason for confusion when buying: two M.2 SSDs can look identical on the outside and still deliver completely different speeds, depending on whether they internally speak the older SATA protocol or the considerably faster NVMe protocol over PCIe lanes.
Whether a particular M.2 slot on the motherboard supports NVMe, SATA or both additionally depends on the motherboard itself and its chipset — the same physical connector shape (the so-called “M-key” or “B-key”, the notch in the connector edge) can pass through different protocols depending on the board. Because of the compact, cableless design, which needs neither a separate data cable nor a power cable like classic SATA drives, M.2 has become the standard form factor for laptops and modern desktop PCs, where saving space and easy installation are important advantages.
PCIe generations and lane count
Besides form factor and protocol, a third factor plays a role for NVMe M.2 SSDs: the PCIe generation (e.g. Gen3, Gen4, Gen5) and the number of lanes used (usually 4 for M.2 SSDs). Every new PCIe generation roughly doubles the bandwidth per lane compared to the previous one — a Gen4 SSD in a motherboard slot that only supports Gen3 automatically runs at the slower Gen3 speed (backward-compatible, but without the full speed advantage of the newer SSD).
Thermal throttling on M.2 SSDs
Because of the compact design without a large enclosure of their own, NVMe M.2 SSDs can get noticeably hot under sustained load and then throttle their own speed (thermal throttling) to prevent damage — similar to a CPU. Many motherboards therefore ship with small heatsinks (“M.2 heatsinks”) directly for the slot, which can make a noticeable difference especially for very fast Gen4/Gen5 SSDs under sustained high load.