XGMII, XAUI, XFP and XFI module and interface specification

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A number of abbreviations in 10G Ethernet start with an "X" letter. This is due to the Roman number 'X', which stands for '10'.

Ethernet defines a clear interface between the MAC block (common to all devices) and a PHY, which is media dependent. This interface is called "Media Independent Interface" and abbreviated with "MII". Gigabit Ethernet consequently named its media independent interface "GMII". Hence the name of the 10 Gigabit Ethernet version - "XGMII".
The 10 Gigabit Media Independent Interface (XGMII) has been standardized in IEEE Std. 802.3ae-2002 and is the interface between the MAC and PHY component. However, this interface can optionally be extended by a so called "XGMII EXTENDER SUBLAYER (XGXS)", which introduces an intermediate "10 Gigabit Attachment Unit Interface (XAUI)".
The XGMII consists of 72 signal lines and is suitable for short range on-board component interconnection.

The XAUI (10 Gigabit Attachment Unit Interface) is the optionally inserted transparent interface, which multiplexes some of the XGMII signals resulting in a smaller line count (16) and increased operational distance. Sinces both ends of this interface will have to deliver XGMII signals, there is a certain clue logic required that does the serialization / deserialization (incl. 8B/10B encoding) to match the 16 XAUI signal lines. The overall speed remains at 10Gbps as required by the XGMII.

XFP and XFI are not standardized by the IEEE and concern the "specification for a module, cage hardware, and IC interfaces for a 10 Gbit hot pluggable module converting serial electrical signals to external serial optical or electrical signals" (source: XFPMSA Group). These modules support OC192/STM-64, 10G Fibre Channel, G.709 (OTN network interface definition OTU-2), and 10G Ethernet, which allows for broad application and reduced module costs.

XFP - the "10 Gigabit Small Form Factor Pluggable" has been standadized by a "Multi Source Agreement (MSA) Group" of many distinguished companies (The XFP is a development activity promoted by Broadcom Corporation, Brocade Communications Systems, Inc., Emulex Corporation, Finisar Corporation, JDS Uniphase Corporation, Innovation Core SEI, Inc., Maxim Integrated Products, Ciena Corporation, Tyco Electronics Corporation, Velio Communication, and many contributors.) working in this field. A free download of the XFP specification is available from the XFP MSA Group website.
XFP modules can be electrical-to-optical or electrical-to-electrical transmission modules. The mechanical and board specifications (module, clip and connector dimensions) are constant for all applications. The most tricky part of the specification is the serial electrical interface.

XFI - the "10 Gigabit Serial Electrical Interface" "... is the high speed serial electrical interface for XFP modules with a nominal baudrate of 9.95-11.1 Gb/s. XFI connects a serial 9.95-11.1 Gb/s SerDes to a module over 300mm of improved FR4 material or up to 200mm of standard FR4 with one connector. The electrical interface is based on high speed low voltage AC coupled logic with a nominal differential impedance of 100 ohm." (source: XFP specification)

A selection of XGMII, XAUI, XFI supporting chips / chipsets can be found in the "10G Ethernet Chips" section of this NG Ethernet Forum.
This brief explaination should soon be extended. Fell free to add your knowledge to this post.