PCI Express has almost completely replaced the long-in-the-tooth PCI bus system in current PC designs. There are still PC motherboards which offer slots for both systems, but due to cost and performance considerations current boards are equipped almost exclusively with PCI Express slots.

PCI Express is not a traditional PC bus system in which all plug-in cards are in parallel with the majority of bus lines. PCI Express is rather a star type wiring technique using a point-to-point connection between the PC and plug-in card to allow significantly greater transmission speeds than a bus solution with parallel wired slots would be able to process.

Data transmission to the plug-in cards in PCI Express is serial using differential wire pairs, so-called "lanes", which run at a speed of 2.5 or 5 GB/s. For cards with high data throughput (graphics and network cards, hard drive controllers), multiple lanes are routed to a slot, whereas cards with less complexity get by easily with one lane.

The PCI Express standard enables solutions in which one (x1), four (x4), eight (x8) or 16 (x16) lanes are routed to a slot, with the connectors differing in the mechanical configuration, pin configuration and load capacity of the supply voltage.

The mechanical coding of cards and connectors ensures however that short cards can be easily used in long slots. For example, a x1 card can be used in a x4, x8 or x16 slot.