The wiki article at
http://www.guildofwriters.org/wiki/Playing_Together should at least answer some of your questions.
To understand the differences, you need to learn a bit about the various versions of Uru:
http://www.guildofwriters.org/wiki/Uru: ... Chronicles (DI, UAM, TOC Shard)
http://www.guildofwriters.org/wiki/Myst ... :_Uru_Live (Gehn, Minkata, TLCA Shard)
For the details, I can only speak about the "old-style" Shards for Uru:CC (often referred to as
TPOTS, too): DI, UAM and TOC are very similar in that they use the Offline KI and provide almost all of the fan-ages. I have no idea how up-to-date TOC is, and while UAM is unmaintained since November 2011 (so you will not find the very latest age updates there), the Deep Island Shard provides up-to-date versions of all ages, as far as I can tell (please email me if there is an update I missed!). TOC and UAM require dedicated registration, DI uses the GOW account database.
However, I did not visit any of these Shards (besides DI), so this is taken from announcements and reports I read.