AAX is the new unified plug-in format that comes in two flavours - AAX DSP, which requires a Pro Tools|HDX system, and AAX Native, which can be run natively on the host CPU with any Pro Tools 10-based system (including Pro Tools|HD Native, the Mbox family, and Eleven Rack) or standalone software. The AAX format provides better workflows and sonic parity between DSP-accelerated and native Pro Tools systems. RTAS and AudioSuite plug-ins will also run on Pro Tools|HDX systems, but TDM plug-ins are not compatible.
AVID created AAX for the future, as it's a 64-bit ready format, and will allow for third-party developers time to ready their plug-ins for Pro Tools technology advancements down the road.
What is the difference between AAX Native and RTAS plug-in formats?
While both are native plug-in platforms, AAX Native uses a new SDK that is 64-bit ready. Pro Tools 10 software will run both versions, but AAX is the Pro Tools platform for the future.
Are all third-party manufacturers updating their plug-ins to AAX format?
Many third-party manufacturers have transitioned (or are in the process of transitioning) their plug-ins to the new AAX format. For the latest details, see the AVID article linked here.