Delta simply means a difference in Ct value, which is what you actually want to know. When you retrieve a Ct value for a certain sample, you compare it with (multiple) housekeeping genes (normalization), hence Delta Ct. however, many times, you compare different treatments, e.g. treated samples vs. untreated sample.

In this case you compare to samples that have already been normalized:

Delta ( DeltaCt (treated) - DeltaCt (untreated))

Basically, it depends on the design of your experiment.

You might find a better explanation here:

http://toptipbio.com/delta-delta-ct-pcr/

Article Analysis of Relative Gene Expression Data using Real-Time Qu...

Good luck!

Thanks