One of the key strengths of the PC is its open architecture and adaptability, historically facilitated by any number of expansion buses that let you add and upgrade certain capabilities through the use of compatible cards.
Old timers (like most of the folks writing for Tom's) will remember the eight-bit Industry Standard Architecture (ISA) bus, developed by IBM in 1981. In 1984, the superior 16-bit IBM AT bus was implemented. Sixteen-bits wide and operating at 8 MHz, it had a maximum theoretical bandwidth of just 8 MB/s.
The next major step in the evolution of the expansion bus came in 1993 with the Peripheral Component Interconnect (PCI) bus. Thirty-two bits wide and clocked at 33 MHz, PCI could move up to 132 MB/s, and it was the first bus conceptualized with plug-and-play in mind.
In 2004, we were introduced to PCI Express (PCIe). We've now reached the third generation of this standard, and PCIe 3.0 is capable of 8 GT/s. Just one lane of third-gen PCIe theoretically moves about 125 times more data per second than the 16-bit AT bus. Implemented as a 16-lane slot, PCI Express can move up to 16 GB/s in each direction.
Early PC motherboards offered very little in the way of integration, so all input and output relied on appropriate add-in cards. Over time, the industry built hard drive, USB, audio, and network controllers into chipsets and onto small-enough components that the motherboard industry could enable all of that connectivity. As lithography evolves and more features find themselves on the processor or closer to it, enthusiasts typically drop in fewer upgrades. But technologies like SLI and CrossFire demonstrate that it's still possible to populate every slot of a modern PC, emphasizing the continued importance of expansion.