In a nutshell. DD (Dolby Digital [aka: AC-3]) is Dolby's original consumer digital system (they have several analog systems that predate) that can do 1 to 5.1 channels of lossy compression. DD+ (Dolby Digital Plus [aka: E-AC-3]) is a newer codec that is more efficient (can be smaller files) at encoding and can do 1 to 7.1 channels.
Also DD+ can be worse or better depending on data rate used and compared. At the same data rate with the same number of channels using the same master DD+ would be better. However you have no way of knowing this and many services (like VUDU) use DD+ to send at a lower data rate (saving bandwidth). In most cases it will be audibly and subjectively the same (especially since you can't DBT them).
Also DD+ can be worse or better depending on data rate used and compared. At the same data rate with the same number of channels using the same master DD+ would be better. However you have no way of knowing this and many services (like VUDU) use DD+ to send at a lower data rate (saving bandwidth). In most cases it will be audibly and subjectively the same (especially since you can't DBT them).
