PCIe switch
Beginner
A chip that lets several devices share one connection to the processor, like a power strip shares one wall socket.
Novice
A chip with one upstream port toward the CPU and several downstream ports toward devices. It lets more devices attach than the CPU has lanes for, and lets devices behind it send data straight to each other.
Expert
A set of logical PCI-PCI bridges. It fans out lanes at the cost of oversubscribing the upstream link and adding some latency, and it routes peer-to-peer traffic (GPU↔NIC, NVMe→GPU) without touching the root complex when access control allows.