AutoCAD blocks
A block is an object that combines one or more single entity object. Operated as an individual complete object, a block can be inserted into a position at a specified scale and rotation angle.

AutoCAD layers
A layer is a powerful tool for users to organize and manage graphics. All graphical objects have four basic properties, namely, layer, color, line type and line width. Users can use various layers, color, line types and line width to draft different objects and elements, effectively controlling the way that objects are displayed and edited, and increasing the efficiency and accuracy in drafting complex graphics.

Relationship between AutoCAD blocks and layers 
Blocks are like original files available for use, but can’t be decomposed. We can apply them in any layer.