Often you see graphic assets entries on the Asset Store, or developers and artists speak about PBR ready scene and PBR textures (materials). PBR stands for Physical Based Rendering and is a modern technique where the final look of a scene will be hit not just by a generic directional light in one direction but also take into consideration real physics light phenomena such as light refraction, specular reflection, and lights rebounding. PBR is achieved in Unity through real-time illumination and GI with lights hitting meshes in a scene that use the Standard Shader.
Unity's Standard Shader is very flexible and you can achieve almost any kind of surface by properly setting the Albedo texture, which is the color texture and, when needed, a normal map texture for bumped surfaces and you can specify Opacity maps, metallic or alpha shining parameters, and much more. Usually artists will provide meshes for Unity that are ready for these kind of materials along their artwork. For example, a modern character model would usually come with Color, Normal, and Specular texture maps to allow the proper rendering of all parts of the character: the skin, where it shines or is more opaque, the clothes, metallic parts on their gear, clothes, rubber, leather, and so forth.
There is also a specular version of the same shader called Standar Shader (specular) available for rendering special shining objects, such as metal, glass, and light-emitting objects, in a different manner. Check out both shaders and try one of your materials with each to see the difference. With the last 2017.3 release, Unity introduced two new shaders meant for particles: the Standard Surface and the Standard Unlit shaders.
Official manual link: https://docs.unity3d.com/Manual/shader-StandardShader.html.
With the requirements for photo-realism and special surfaces, such as semi-transparent rocky surface (salt), gel, or ice/snow, Unity's Standard Shader might be not enough. You might want to check out the Uber Standard Shader ultra, which is like Unity's Standard Shader on steroids. You can find it on the Asset Store at https://assetstore.unity.com/packages/vfx/shaders/uber-standard-shader-ultra-39959.