Promis.e shared cells are similar to AutoCAD blocks.
A shared cell is a cell whose elements are stored only once in the DGN file, regardless of how often the cell is placed within the DGN file. Any change made to one instance of a shared cell reflects in all instances of that shared cell. For an unshared "normal" cell, on the other hand, the library definition is stored in the DGN file each time the cell is placed. In AutoCAD, all blocks behave like shared cells. There is no analogy to a normal cell.
Some Web pages contain DWG content that is i-Drop enabled. This means that you can drag the content from the Web page into a Promis.e model. When you drop this content into a model, it is placed as a cell.