From the Cakewalk website...
"DXi is the new software synthesizer plug-in standard for Windows-based music production.
Developed by Cakewalk and based on Microsoft DirectX technology, DXi is an open standard available to software developers for creating DXi plug-ins and host applications for the PC.
DXi soft synths make it possible to recreate the sounds of vintage analog synth gear, as well as create entirely new sounds and combinations from within professional Windows applications like SONAR. It brings a new level of soft synth performance to the Windows platform, along with new levels of creativity for recording musicians and producers.
Companies supporting DXi
Leading soft synth developers have announced support for the DXi standard. The navigation bar to the right links to information on third-party DXi soft synths that will be available for DXi host applications. See the respective manufacturer for more information and availability of individual software synthesizers. Where applicable, the manufacturer's logo links to their web site.
Current DXi-compatible hosts:
SONAR
SONAR XL
Home Studio 2002
Music Creator 2002
Plasma
Fruity Loops 3
Infinity
Orion Pro"
All of the above as opposed to VST plugins which are probably the most pervasive and which can also be used within the above hosts too. However, to use VST in the above products you would have to get a peice of software called a "wrapper" which translates from VST to the interfaces for the above apps. Some native VST apps are Cubase, Vegas etc.
Now if you use Pro tools, they have their own proprietary format and that's about all I'll say about this because I have never used it and my knowledge comes from reading posts in this arena.
Hope this helps and if I missed anything, anyone please comment.