I'm as new to this topic as you are.
In computer graphics, rendering has been an area that I'm particularly interested in; apart from effects & compositing. Only recently I have been forced to learn and pick up rigging (whoa!) after my friend Hilwan got me not one but two Art of Rigging books; a real hardcore dig-in into the complexities of rigging. Well, I'm sharing this post with you in the hope it could inspire some creative nuances.
 
Rendering in passes is the process of rendering different attributes of your scene separately, such as the different pass types below. In Maya, you will notice on the right side, bottom, there are layers panel similar to Photoshop. The layers function apart from activating visibility of objects in the scene also allows multi-pass rendering; that means you are allowed to render one layer at a time.  
 
Rendering can be time consuming and CPU power draining; hence rendering in layers becomes a definite solution.
Here I will share with you some rendering terminologies used (courtesy of the lighting guru Jeremy Birn).
 
BEAUTY PASS - A beauty pass (sometimes called diffuse pass or color pass) is the main, full-color rendering of your subject, including diffuse illumination, color, and color maps. A beauty pass usually will not include reflections, highlights, and shadows, which are usually separate passes.
 
In the example below, the model spaceship is placed against the background of stars hovering over another big planet below it. The beau pass of the spaceship alone does not give a visual impact to the whole scene.
 

 
Hence, something else needs to be done to pimp up the look. That's where other passes or render layer have to be added to enhance the visual outlook.
 
SPECULAR PASS - Highlight passes (sometimes called specular passes) isolate the specular highlights from your objects. You can render highlight passes by turning off any ambient light and making the object’s diffuse shading and color mapping to pure black. The result will be a rendering of all the specular highlights in the scene, over a black background, without any other types of shading.
 

 
REFLECTION PASS - A reflection pass includes reflections of other objects or the surrounding environment, and can either replace or complement the highlight pass. To isolate reflections, usually all you need to do is turn off ambient, diffuse, and specular shading from a surface, so that only reflections appear.
 

 
SHADOW PASS - A shadow pass is a rendering that shows the locations of shadows in a scene. A shadow pass often appears as a white shadow region against a black background, a black shadow against a white background, or a rendering with the shadow shape embedded in the alpha channel. Cast shadows are where an object casts a shadow onto another 3D object or darkens an area of a live-action plate. Separate shadow passes can depict attached shadows, where an object casts shadows onto itself, such as the shadow a character's nose casts onto his own face.
 

 
LIGHTING PASS - A lighting pass is an optional part of multi-pass rendering, that adds flexibility and control to the compositing process. Instead of rendering a beauty pass all at once, you could instead render multiple lighting passes, with each individual lighting pass showing the influence of one light (or one group of lights) on an element. Other lights are hidden or deleted when rendering the lighting pass.
 

 
OPTIONAL PASSES
 
Effects passes may sometimes be rendered, depending on the needs of your project. An effects pass is a separate rendering of a visual effect or a mask for a visual effect. An effects pass might be an optical effect, such as a light glow or lens flare, or a particle effect, such as a cloud of smoke or plume of jet exhaust.
 
A depth map (also called Z-Depth or a depth pass) is a pass that stores depth information at each point in your scene. Some productions use depth maps rendered in a special depth map file format. Other productions use simulated depth maps which are rendered as standard image files just like any other pass, but with a depth-fading effect over objects with constant white shading.
 

 
In the above pic, I added smoke generated from the landing spaceship, motion blur to suggest some movement and white glares.
 
Passes can be rendered one at a time by rendering differently modified versions of your 3D scene, or some software can set them up automatically or render more than one pass type at once.
 
Thank you for reading this - lol  
 
- Remy Mahzam