Shading and texture can really add some flare into your artwork!
I want to show you quick and basic differences shading and textures make to your artwork
Shading makes your picture look more 3D and realistic
Sharp edged shading will make your drawing look more edgy & cartoony while blended shading will make it look more soft and relaxed (well...that depends on colour as well)
Texture makes your picture more interesting
When you have textures incorporated into your artwork it makes your picture interesting! Depending on which texture you use, it can also hide small mistakes such as wiggly lines. This is because when you have a texture, people aren't going to be directly looking at the basics anymore since your image has more details.
Here is an example of a flat image which I coloured for a contest:
Lines are by Tavvy on DeviantArt
And here is the image after I finished shading and giving texture to it through different brushes:
There you go! Some people like the flat image as opposed to the shaded one, and others like the shaded and textured picture over the flat picture. It is really up to your own personal tastes!
If you have any questions, feel free to ask me! (Im not a pro so my answers will probably be based on my own experiences though)