From BlenderWiki
After blender version 2.72, projective texture painting workflow has been revamped to make it easier for users to setup and use the system.
Projective Texture Painting
Blender mesh objects have a mode, called Texture Paint mode that allows artists to directly paint on image textures attached to the material of the object.
The basic interaction is done through blender's brush system (insert link here), through which artists can apply strokes on the mesh's surface by using the mouse or a pressure sensitive tablet for more control.
Apart from the brush stroke system, which is mostly common among all of blender's painting modes, there are texture painting specific tools and options.
Blender's new texture paint system used to operate on images assigned to the meshes UV layers. However the new system, uses the materials assigned to the mesh.
The texture painting system currently supports material slots that of image type using UV mapping. If no such texture image, uv layer or material exists, blender takes care of creating one for the user, however no optimal uv unwrapping takes place which means all faces share the same texture space. It is strongly adviced for users to first make an optimally unwrapped UV layer before entering texture paint mode.
The available texture paint slots can be seen in the Layers tab. If the mesh has more than one materials assigned, there's also a material selection so the active texture paint slot for each material can be selected.
