Alpha textures are textures with transparency. When applied to 3D models, they make areas of the model transparent, allowing objects behind it to show through. Alpha textures can be either TGA files with dedicated alpha channels or PNG files with transparent pixels. In either case, alpha textures can render with artifacts in Unity if they're not created and imported correctly:
Creating alpha textures
If you need to use alpha textures, ensure that you check out the official Unity documentation on how to export them for optimal results at: http://docs.unity3d.com/Manual/HOWTO-alphamaps.html.
