These are x16 and x1 PCI Express slots.

Obviously, bandwidth cannot be a serious reason for moving from PCI-X to PCI Express in the professional market, because PCI-X has plenty. Thus, there must be other reasons to attract professional customers. Let's start by taking a look at current PCI Express speeds.

As you can see in the image at the top of this page, PCI Express x1, which is the simplest configuration, requires very few connectors and still achieves a bandwidth that exceeds the capabilities of 33 MHz 32 Bit PCI by at least a factor of two. The reason for this is its serial operation - rather than transferring one bit per pin as is the case with PCI(-X), PCI Express uses two low-voltage differential signalling pairs that run at 2.5 Gbit/s each. PCIe uses an encoding method that takes 10 bits of signal for every 8 bits of data, which explains why 2.5 Gbit/s per lane results in 250 MB/s rather than 312.5 MB/s (as it would be with 8 bit transferred per 8 bits signalled). Obviously the low pin count can be an advantage for motherboard designs, since routing becomes easier.