I want to know the release naming conventions in linux.. I know the following...
2.1.xx --> means developmet release
2.2.xx --> means stable release...

OK for now.

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Now what 2.2.xx-pre6 or 2.2.xx-ac3 means ??

The 2.2.XX-preY pre kernels are basically pre-releases, which means getting fairly close (except for a few bugs) to the 2.2.XX kernel. Of course, the highest the Y the closest to the release, and this applies to other branches, such as 2.4.
The 2.2.XX-acY are the "Alan Cox releases". All the patches submitted to this list are merged by Alan, then released as patches (you can get them from ftp.KK.kernel.org/pub/linux/kernel/people/alan/2.2 or 2.4, where KK if your country code {uk|de|fr|whatever}). They can be considered as some sort of intermediate steps between two releases, although some of the patches may not be included in the next release, for different reasons. The current one for the 2.4 branch is the 2.4.3-ac2.

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Amol

Best regards
Dantes