Get a named texture.
Many shaders use more than one texture. Use GetTexture to get the propertyName texture.
Common texture names used by Unity's builtin shaders: 
"_MainTex" is the main diffuse texture. This can also be accessed via mainTexture property. 
"_BumpMap" is the normal map. 
"_Cube" is the reflection cubemap.
See Also: mainTexture property, SetTexture,  Shader.PropertyToID.