2D skins / 3D Objects
You can customize your ingame character with a 2D skin which change his looks but not its 3D model. You can find models and skins to use on ManiaPark for example.
You can find a tutorial for creating your own here (http://forum.maniaplanet.com/viewtopic.php?f=321&t=4372) or here (http://wiki.maniaplanet.com/en/Create_a_playerskin).
3D Objects are assets that you can add and use in the ingame map editors into custom titles.
To create an object (for example a block for ShootMania) you need a software like 3DS Max or Blender.
The specifications to create an 3D object is available in this link : http://forum.maniaplanet.com/viewtopic.php?f=321&t=13673 (and for a skin : http://forum.maniaplanet.com/viewtopic.php?f=321&t=4372)
Then you have to run a command line to import it ingame with the Nadeo Importer:
NadeoImporter.exe Object <3dFileNameRelativeToWorkBlocksFolder>
Have fun creating a map with your new object.