3D Modeling is a creative process where you can build a model (character, creature, vehicle, etc.) in three dimensions (x, y, z) as opposed to simply drawing or painting a picture in two dimensions.
There are tons of different software options available to create 3D content and they vary in terms of complexity, price, and system requirements.
If you're new to 3D, try these free, easy-to-learn tools:
3D Slash: Use cubes to build printable models
ZBrushCoreMini: Digital sculpting for beginners, from the makers of ZBrush
For more advanced 3D features, check out some software popular among our users:
3ds Max: Modeling and rendering (Windows only)
Blender: Modeling, rendering, animation (Free, open-source)
Maya: Modeling and animation
Modo: Modeling, sculpting, 3D painting, animation and rendering
Rhino: Mathematically precise, freeform surface modeling
SketchUp: Modeling with a focus on ease of use (Free)
Unity: Cross-platform game engine (Free)
ZBrush: Sculpting, texturing and painting
There are many, many other great choices out there. Even some video games have high levels of support for exporting 3D models, including Minecraft and Spore.
Many platforms also have built-in or plug-in support for Sketchfab publishing; check out our Exporters.