So, it's time to ask some questions about it...
What's it all about?
Where the musical element is perhaps similar to its predecessor (House, if you couldn't guess), the structure is where it differs. Club-goers have become more hooked on those epic tracks that take ages to build and reward you with an intense climactic chorus that you cannot help but dance to.
I've noticed a tendency for authentic sounding instruments hovering over the funky, gritty basslines as well - strings tend to be popular, with artists such as Justice and Deadmau5 using them to great effect.
When did it start?

The later 90s and early 00s brought more definition to the genres and Progressive House started to become more and more commonly accepted in its own right.
How did it start to get popular?

Why is Progressive House so epic?
It's the combination of making people wait for that all important climax with the huge, hooky instrumentation. Strings and Piano are so good at making powerful music when used effectively, and many artists do just that.
High production values from the best of the best in the genre push other producers to match it and it's definitely helped to move electronic music into a new era of sound quality.
Where can I get it?

Wanna start making big Progressive House tracks right away ? Then check out Prime Loops' powerful "Progressive Synthesis" sample pack, providing you with tons of cool and authentic synth patches for Kontakt, EXS24, Halion, NN-XT and many other sampler instruments!