On some sheet create a listing down a single column of all available valid entries. You will be able to add and delete from this list as needed.
For this, I'm assuming the list is Text values and is entered in Sheet2, column A starting in row 1.
Now click Insert | Name | Define
In the Names in workbook box, enter the name you want to refer to this list by - cannot include any spaces. I'll assume here you chose MyList
In the Refers to box, enter:
Now go to the sheet and cell in which you want to restrict data entry to the choices from that list and select the cell.
Now click Data | Validation
In the allow box, select List.
In the Source box, enter: =MyList
Check ignore blank, check In-cell dropdown.
On the Error Alert tab, select Stop in the Style box. Add any titles/messages you want on that tab and on the Input Message tab.
From now on, your entry will be restricted to values from Sheet2 column A.