Before starting the explanation of the more complex aspects of textures, we should simply say that a texture is an image applied to a material. Imagine a glossy wall in a room. You will probably have some lights in the room and even a big window from where the indirect light of the exterior comes in.
As our wall is glossy, it will reflect the lights and even the environment a little bit. But there's more. You can put some textures all over it. You may also want the wall to have some cool vinyls pasted on it. You can have that and still retain the material's properties. You just need to google for a good image to be used as a texture (or you can paint or even click a picture if you have the skills or don't find what you're looking for). Note that googling images is only right if those images are copyright-free. Then, you can paste the image on the wall's surface to get a nice glossy wall with some vinyls on it.
Keep in mind the following main points:
Material: How the surface...