Render Terminologies:

Render Layer:
Mechansim allowing segment 3D scene into fragments for rendering purposes, like extract shading information for compositing purposes.

1) Isolate a foreground character from its background (environment)

2) Isolate an element of a character like Hair, Fur, Cloth, Skin surfaces

3) Isolate a prop, like a weapons, elements on a table, glass water on a table, etc.

Render Pass:
It allows extracting shading elements, for example digital illumination produces several components like diffuse, specular, reflection, refraction, etc. All these components are calculated seperatly and mathematically re-assembled to create a single output value, so called Beauty Pass. Render pass is a mean to "store" those shading components seperately and allow to write it to disk in its purest form. These components are being produce at render time and can contains structure information like (color, float, vector). and write it to disk seperately. Those seperate render passes file can be re-assembled by a compositing system like Toxik.

Channel:
An image is composed of pixels, in a row/column form. Each pixel contains color information in a Red, Green, Blue definition, where Three values are defined per pixel. Those values are called The Red Channel, the Blue Channel, and the Green Channel. Each channel has a commonly used name like "R" for Red, "G" for Green, "B" for Blue. Also another extra channel can be provided with an image, which is called an "Alpha" channel, which contains opacity information for each of the pixels. It is important to mention some file format can host different type of "channels", other than RGB or RGBA. For instance depth information, mathematical construct for compositing like surface direction, normal map, UV, UVW are very commonly produced as "extra channels" to help the compositing process.

Thanks to Dave Lajoie for his tip