What is Dry Hopping?

A great way to amplify the flavour profile from the hops without extracting more bitterness or creating a higher alcohol content as seen in a double IPA. While wet hopped refers to beers made from seasonal, fresh harvest hops; dry hopping is about the timing of the hops additions. And much like steeping a teabag in tepid water, it takes a few days.
When using this technique, brewers wait till the wort has cooled before adding hops. In doing this, the aromats are a lot fresher, and quite different to what you'd expect to see from a beer with hop additions made before boiling the wort, or after when it's still hot. Essentially, when hops are included in the boiling process, the bitterness (IBU) increases, and the aromats get cooked off.
Dry hopping can be done multiple times or with an increased amount of hops - depending on the individual brewer's definition of Double Dry Hopping. Like a lot of beer terminology, it isn't locked in to strict definitions.

But wait - there's more.
A Tassie hops company called HPA has developed a hops product called Spectrum. It's 100% hops, but in liquid from. This super concentrated syrup is a breakthrough in creating a more quantifiable and consistent dry hopped flavour. There are now a few more hops suppliers with similar products.

A couple of beers that are really showcasing Spectrum are the Bridge Road New Sky Vol2. One Drop 3022 Synthetic IPA and the Tallboy and Mouse Continuum.