PCI

The name PCI has been derived from Peripheral Component Interconnect which describes a set of industry standard computer bus architectures which are used to connect components on the computer main board to each other, and also provides an expansion bus to install add-in cards.

PCI interface is divided into 32bit and 64bit, 32bit is the ordinary PCI interface which used of in the desktop, 64bit is longer than 32bit, which only used in server, 32bit and 64bit have two kinds of voltage: 5V and 3.3V. 5V is PCI2.1 standard, clock frequency of 33MHz, 3.3V is PCI2.2 standard, and it can work in the clock frequency of 66MHz. But now, in general, cards and slots are made can be compatible with both of voltages, and also have the design of error proofing, they can work as long as plug it, but in which kind of clock frequency is necessary to analyze, 32bit PCI interface is widely used, even the latest motherboard will stay a few slots for it, but 64bit PCI interface seems to be basically eliminated on the server.

2. 64Bit PCI

64Bit PCI is longer than 32Bit, it is divided into 5V and 3.3V, 

PCI-X

PCI-X is an approach to increase the maximum transfer rate beyond the maximum rate achievable by conventional PCI. Since this is mostly a requirement for expansion cards used in servers, e.g. network or hard disk controllers, PCI-X slots can mainly be found on special server mainboards.

The appearance of PCI-X is same as PCI, but they are using different standards, PCI-X slot can be compatible with PCI card (through the pin to distinct), PCI-X is shared bus, plugging multiple devices will slow down the transfer rate. PCI-X generally only appears on the server motherboard, but now gradually replaced by PCI-E, many manufacturers have not provided the server's PCI-X slot.

PCI-E

PCI Express is the new generation of bus interface proposed by Intel, PCI Express uses the industry's most popular point-to-point serial connection. Each device has its own dedicated connection rather than the shared parallel architecture of the PCI and earlier computer buses, which does not require bandwidth to the entire bus and can increase the data transfer rate to a high enough frequency that PCI cannot provide. Compared to traditional PCI bus can only achieve unidirectional transmission in a single period of time, PCI Express dual simplex connection can provide higher transmission rate and quality. PCI-E slot is backwards compatible, such as PCI-E 16X slot can be inserted 8X, 4X, 1X card. Now the server will provide multiple 8X, 4X interface, which has replaced the previous PCI-X interface.

PCI, PCI-X, PCI-E specifications 

Standards	Bus	Clock	Transmission rate
PCI 32bit	32bit	33MHz 66MHz	133Mb/s 266Mb/s
PCI 64bit	64bit	33MHz 66MHz	266Mb/s 533Mb/s
PCI-X	64bit	66MHz 100MHz 133MHz	533Mb/s 800Mb/s 1066Mb/s
PCI-E X1	8bit	2.5GHz	512Mb/s(Duplex)
PCI-E X4	8bit	2.5GHz	2Gb/s(Duplex)
PCI-E X8	8bit	2.5GHz	4Gb/s(Duplex)
PCI-E X16	8bit	2.5GHz	8Gb/s(Duplex)

Summary

1. 5V PCI and 3.3V PCI/PCI-X slot connectors are keyed differently in order to prevent cards which don't support the same signal level from being inserted into that slot. 

2. Many PCI add-in cards can work with either 3.3V or 5V signal levels, so they are keyed for either of the slot types. 

3. If a conventional PCI card is installed in a PCI-X slot then the clock speed of other PCI-X slots may be reduced, see this note for details. 

4. PCI Express is a totally new approach, so PCI Express cards can neither be installed in conventional PCI or PCI-X slots, nor can conventional PCI cards or PCI-X cards be installed in a PCI Express slot.