How are 'Pluggable Transport' bundles different from 'regular' Tor Browser Bundles?
Tor Pluggable Transport bundles are the new name for the old 'Pyobfsproxy/Flashproxy' and 'Obfsproxy' Tor Browser Bundles . The PTB's include the python version of obfsproxy (not the old C version), and include more transport modes (see below).
The overall idea behind the PTB's attempt to disguise Tor traffic to look like traffic from other common services (eg. normal HTTPS traffic, Skype traffic, etc. Some of the pluggable transports also transform other aspects of Tor traffic; for instance, the Flashproxy version turns web browsers into relays using normal websockets, or StegoTorus (a fork of Obfsproxy) splits Tor traffic between multiple streams to prevent fingerprinting.
If you don't have a specific use for a pluggable transport bundle (eg. you live in a country that does deep packet inspection to detect or block Tor traffic), you can probably go with the normal TBB.
More information can be found in the docs.