Forgive me calling it "next (technical) revolution", terms too often too hasty used, but sometimes a revolution comes and isn't so fast and only in retrospect the revolution is realized.
Gutenberg in 1439 used "moveable type printing" - where each letter was composed on a grid and so the text was composed, page by page, and so was page for page printed and then the book was composed. It made information in printed form much easier to reproduce.
Gutenberg Metal Moveable Type
Printing in 1880's
The invention of computer adapted Gutenberg's method, and this is why we read text on a screen, the letters are very moveable, even virtual. Printers on the other hand bridge the virtual world with the real world again, but it was like reversing an evolution - more paper was printed due to computers, no paperless office - also the question of authenticity hasn't been yet resolved: printed paper is hard to alter, whereas to alter a computer file is easy.
3D Printed Form
Anyway, the term "3D Printing" kind of is an understatement - it permits to manifest what is done virtual. 3D printers exist since decades, but they have been very costly and complicate to manufacture and use too. The last few years 3D printers have been developed which are able to print parts for themselves, replicate themselves partially - from there comes the term "Reprap" (replication rapid prototyper). Late 2011 and beginning of 2012 brought 3D printers as open hardware and open software at very low cost - plans and programs open and freely available, and the price range reached USD 300 with a printing volume of apprx. 20cm x 20cm x 20cm or 8" x 8" x 8", such as the Printrbot (a Kickstarter.com project which raised 830K USD):
3D Printing Material: ABS (3mm Euro 25 / 1kg, different colors)
What can you print depends on the material you can extrude and print out - for now: ABS (plastic, recycable), PLA (biodegredable), clay, plaster etc. essentially material which can be liquified and solidifies quickly at room temperature.
Plastics such as ABS or PLA etc)
With several extruders one is able to mixed colors or materials - yet not so much research in mixed material has been made yet (status 2012).
Parts for another Reprap
You can print some parts of a Reprap yourself, replicating the machine itself. Motors and electronics and some metal rods cannot be replicated with a Reprap yet, motors and rods could in the near future also replicated - perhaps even the microprocessors; the challenge are the mixed materials at tiny scale.
HDPE: (Milk Jugs, bottles, bottle caps, water pipes)
LDPE: (Trays, plastic wraps, slides)
ABS: (Luggage, water pipes, Lego's)
PLA: (Packaging, organic-waste, bio-degradables)
Building Larger Objects
My personal interest are objects larger than the 20cm x 20cm x 20cm, which means, I focus on extendable designs - means, printing parts which can be assemblied into larger objects, ideally universially.