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PCI Express (PCIe) is a high-speed, serial interface standard used to connect a computer's motherboard to expansion cards and other peripherals. It is the modern successor to the older PCI, PCI-X, and AGP standards. PCIe is the primary internal data "highway" for high-performance components like graphics cards and NVMe solid-state drives (SSDs).

Key concepts of PCIe

Point-to-point architecture: Unlike the shared bus of older standards like PCI, PCIe uses a switch to create a direct, dedicated connection between each device and the CPU. This eliminates bandwidth bottlenecks and minimizes latency.

Lanes: Data travels along "lanes," which consist of two pairs of wires for simultaneously sending and receiving data. The number of lanes is expressed as a number with an "x" prefix (e.g., x1, x4, x16).

More lanes equal more bandwidth. An x16 slot, for example, has 16 lanes and provides the highest bandwidth for data-intensive devices like high-end graphics cards. Lower-lane slots are used for devices like network or sound cards.

Slots: The physical connectors on a motherboard where PCIe cards are installed. A smaller card can fit into a larger slot, but it will operate at the bandwidth of its smaller lane count. For instance, an x1 card can fit into an x16 slot.

Generations: With each new generation (e.g., 3.0, 4.0, 5.0), the bandwidth per lane doubles, leading to much faster data transfer speeds. The standard is maintained and updated by the PCI-SIG, an industry consortium.

PCIe is backward-compatible. A newer-generation card (e.g., PCIe 4.0) will work in an older-generation slot (e.g., PCIe 3.0), but performance will be limited to the maximum speed of the older slot.