Blocks and groups both combine other drawing objects into a single object that can be moved, deleted, etc. However, they have some very important differences.
When you group objects, you combine separate objects into one object with many parts. If you move the group on the drawing, all the objects in the group move together, retaining their relationship to each other.
Blocks definitions are stored in a file and reproduced in a drawing through a block reference that determines the size, rotation and other properties of the block.
Editing a Block Reference
Using Blocks from Other CAD Programs
Storing Blocks in the Drawing
Converting Blocks to Groups
Grouping Drawing Objects