Materials are very important in a design when doing analysis or creating presentations and marketing materials. For Inventor you have 2 libraries, the Inventor library and the Autodesk Library. These both have the physical properties as well as the ability to make it look like the real world materials that they are modeled after.
To find your materials that come installed with Inventor select the materials browser above the ribbon.
You are able to select materials either from the Inventor library or the Autodesk material library. If the material is not present in these libraries then you re able to create your own materials. This could involve taking pictures of the material or finding a bitmap picture of it online and then manipulating it in the material editor to suite your geometry.
You will notice there are 2 tabs, Appearance and Physical.
These materials properties allow you to manipulate the material with the real world physical attributes like reflectivity, transparency and self illumination.
If you are not familiar with the terminology then the easiest way to create new materials is to find an existing material in the library and maybe swop out the bitmap of the material to one that looks more like the material finish that you are looking for.
If you are looking for more realistic materials and rendering I would recommend going to 3ds MAX or VRED. These have an extended library of materials which greatly increase the awesome look that you can give your models.