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PCI Express is a point-to-point serial connection that operates more like a network than it does a bus. Traditionally, busses handle data from multiple nodes.
PCI Express, on the other hand, employs a controller chip to manage several point-to-point serial connections. This is similar to the way USB works, and the B in USB stands for bus.
A bus is typically a parallel network of devices that all share the same data lines. The name just kinda stuck as we transitioned from slow parallel buses to high-speed serial networks.
What came before PCI Express?
Although the name makes it sound like it replaced the 32-bit PCI slot that came before it, and in a way, it did, but PCI Express was really more of a replacement for AGP than anything else.
It was no coincidence that Graphics cards went from using an 8X AGP slot to a 16X PCI Express link.
So, what replaced PCI then?
PCI Express! lol. Anything that required the PCI bus before can be done with a single (x1) PCI Express link. More bandwidth-hungry devices can use any multiple of PCI Express lanes. Most high-end network controllers will use a card that has a physical x8 edge connector but they typically only use 4 PCI express lanes worth of pins.
Graphics cards, of course, are the most bandwidth-hungry device your computer has, so those will almost always have a 16x PCIe connection.