1. What is 3D printing?
3D printing is also known as desktop fabrication or additive manufacturing, it is a prototyping process whereby an real object is created from a 3D design. The digital 3D-model is saved in STL format and then sent to a 3D printer. The 3D printer then print the design layer by layer and form a real object.

2. 3D printing technologies
Quite a few technologies are capable to do 3D printing. The main differences are how layers are built to create parts.
SLS (selective laser sintering), FDM (fused deposition modeling) & SLA (stereolithograhpy) are the most widely used technologies for 3D printing. Selective laser sintering (SLS) and fused deposition modeling (FDM) use melting or softening material to produce the layers.
This video describe how laser-sintering process melt fine powders, bit by bit, into 3D shapes.
This video shows how FDM works.
The video belows explains the process of Stereolithography (SLA).
Generally, the main considerations are speed, cost of the printed prototype, cost of the 3D printer, choice and cost of materials and colour capabilities.

3. The history of 3D printing
October 5, 2011 - Roland DG Corporation introduced the new iModela iM-01.
Sep, 2011 - Vienna University of Technology, a smaller, lighter and cheaper printing device has now been developed.
This smallest 3D printer weighs 1.5 kilograms, it costs around 1200 Euros.
Aug, 2011 - The world’s first 3D printed aircraft created by Engineers at the University of Southampton.
Read more..
4. 3D printing applications
One of the most important applications of 3D printing is in the medical industry. With 3D printing, surgeons can produce mockups of parts of their patient's body which needs to be operated upon.
3D printing make it possible to make a part from scratch in just hours. It allows designers and developers to go from flat screen to exact part.
Nowadays almost everything from aerospace components to toys are getting built with the help of 3D printers. 3D printing is also used for jewelry and art, architecture, fashion design, art, architecture and interior design.
Here are some extraordinary examples of 3D-printing:

• The World's First 3D-Printed Car

• World's first chocolate printer

• World's first 3D printed bikini

5. What is a 3D printer?
3D printer is unlike of the common printers. On a 3D printer the object is printed by three dimension. A 3D model is built up layer by layer. Therefore the whole process is called rapid prototyping, or 3D printing. Read more..
The resolution of the current printers are among the 328 x 328 x 606 DPI (xyz) at 656 x 656 x 800 DPI (xyz) in ultra-HD resolution. The accuracy is 0.025 mm - 0.05 mm per inch. The model size is up to 737 mm x 1257 mm x 1504 mm.
The biggest drawback for the individual home user is still the high cost of 3D printer. Another drawback is that it takes hours or even days to print a 3D model (depending on the complexity and resolution of the model). Besides above, the professional 3D software and 3D model design is also in a high cost range.
Alternatively there are already simplified 3D printers for hobbyist which are much cheaper. And the materials it uses is also less expensive. These 3D printers for home use are not as accurate as commercial 3D printer.

6. Whats the difference between a basic rapid prototyping machine and a 3D printer?
3D printers are the simple version of rapid prototyping machines. It is lower lost and less capable.
Rapid prototyping is a conventional method that has been used by automotive and aircraft industries for years.
In general 3D printers are compact and smaller than RP machines. They are ideal for use in offices. They use less energy and take less space. They are designed for low volume reproduction of real objects made of nylon or other plastics. That also means 3D printers make smaller parts. Rapid prototyping machines have build chambers at least 10 inches on a side, a 3D printer has less than 8 inches on a side. However a 3D printer is capable of all the functions of rapid prototyping machine such as verifying and validating design, creating prototype, remote sharing of information etc.

Consequently 3D printers are easy to handle and cheap to maintain. You can buy one of those DIY kit in the market and build up yourself. It is cheaper than the professional rapid prototyping, for $1000 or less you can have one 3D printer. While the professional rapid prototyping cost at least $50,000.

3D printers are less accurate than rapid prototyping machines. Because of its simplicity the material choices are also limited.

7. What can you make with a 3D printer?
In 3D printing area, people say "If You Can Draw It, You Can Make It". In the video below shows many items can be made with a 3D printer. However complicated objects can only be made by professional 3D printers, they are not yet affortable for common family.

8. Who make 3D printers?
Industrial 3D printer manufacturers:

• Stratasys
• 3DSystems

Home 3D printers manufacturers:

• Reprap.org 
• Makerbot Industries
• Ultimaker
• Fab@Home

9. How much cost a 3D printer?
Here are list of DIY 3D printer price comparison and list of fully assembled or commercial 3D printers price comparison (less than $25,000).

10. How to build a 3D printer?
The MakerGear Mosaic 3D Printer
Part I: The Frame
Part II: The Y-Axis
Part III: The X-Axis
Part IV: The Z-Axis
Part V: The Extruder
Part VI: The Build Platform
Part VII: The Electronics
Reprap
How to build a reprap 3D printer - RepRapOneDarwin (1st generation)
How to build a reprap 3D printer - Huxley (mini-reprap, portable)
How to build a reprap 3D printer - Mendel (RepRap Version II)
How to build a reprap 3D printer - Prusa (easy to asembly)
More...

11. What are the materials used to print 3D objects?
Many different materials can be used for 3D printing, such as ABS plastic, PLA, polyamide (nylon), glass filled polyamide, stereolithography materials (epoxy resins), silver, titanium, steel, wax, photopolymers and polycarbonate.

12. What 3D modeling software is suitable for a beginner in 3D design?
If you’re just getting started you can try some of 3D modeling software which can be downloaded for free.

• Google SketchUp - This Google SketchUp is fun and free, and is known for being easy to use. To build models in SketchUp, you draw edges and faces using a few simple tools that you can learn in a short time. With with Push/Pull tool you can extrude any flat surface into a 3D form. Furthermore, it works together with Google Earth, that you can import a scaled aerial photograph directly from Google Earth, or use SketchUp to build models which can be seen in Google Earth.

• 3Dtin - The simplest 3D software. You can draw directly from your browser. 

• Blender - Blender is the free open source 3D content creation suite, available for all major operating systems under the GNU General Public License. Blender was developed as an in-house application by the Dutch animation studio NeoGeo and Not a Number Technologies (NaN). It is a powerful program contains features that are characteristic of high-end 3D software. 

• OpenSCAD - OpenSCAD is a software for creating solid 3D CAD objects. It is free software and available for Linux/UNIX, MS Windows and Mac OS X. it does not focus on the artistic aspects of 3D modelling but instead on the CAD aspects.

• Tinkercad - Tinkercad is a new and faster way of creating designs for your 3D printer. With only three basic tools you can create a wide range of useful things. Once your project is ready simply download the STL file and start your 3D print.

Check out the more software in the free software list. 
Commercial software such as CAD software AutoCAD and Pro Engineer, software packages Rhino, Maya, and SolidWorks are all pretty good for designing 3D models.

13.  I have no 3D design experience, how long does it take to learn 3D modeling?

15. Where can I find online 3D printing service?
Companies like Shapeways, i.Materialise, Sculpteo and Ponoko provides online 3D printing service. More to find here: List of 3D printing service