If you mean you would like to have a mathematical calculation that sums a group of cells and updates when ever you change onay one of those cells. The you can type a formula like:
In cell A1 type:
"=sum(B1:B10)" this will sum the first 10 rows of column B
"=sum(B)" this will sum every row of column B
there is another way:
You can highlight the cells you want to sum, then drag the "SUM" tag from the bottom left into the cell where you want the result to reside.
I don't think autosum in Excel works the same way as Wayne is suggesting. In Excel, you can be in a cell at then end of a column and hit the autosum button. It will automatically add the cells until there is an empty cell above it. I hope that makes sense - it saves a bunch of time, because you don't have to type in "=sum( etc"