Merging Options available in Excel

From above screen there are multiple options available for Merging the excel cell.
Merge & Center - This option allows you to Merge the multiple                                     cells and keep the contents in the center of the                                   cell.

Merge A1 and B1 


Merge Across     - Merge the selected cells in to larger cells while                                 keeping rows separate.

Merge Cells        - Merges selected cell into one cell but does not                                   align the text to the center.

Unmerge Cells   - Unmerge the cells means which allows you                                         reverse the smaller cells from the bigger cell.