Amongst other things, Diffuse map or color part of the bunch of different maps - images is probably the most important one you could ‘glue’ on your object. All you need is blank image and so called UV map which says what part of image comes where on object.
As you see all bumps into UV map.
Let’s assume you have it. It can be exported as an image from blender’s UV editor window and can be further used to assist any other map’s painting in any image editor available.
To further improve object’s look in 3d and determine how it’s surface reacts to the lights thrown at it, several other ‘maps’ - images which are ‘mapped’ using UV coordinates can be used - reflection map, specular highlights map, bump map/maps, normal map, AO map. Each does it’s part to how colors look on object. First two are self explanatory, normalmap - improved bump map; AO - ambient occlusion map which says that creases, dents, gaps look more dark usually.
All of them can be ‘Baked’ - generated in Blender using Blender Render engine (Render tab, down below) and further used in any image editor (see above - UV map part).