An active worksheet is the worksheet that is currently open. For example, in the picture above, the sheet tabs at the bottom of the window show "Sheet1", "Sheet2", and "Sheet3", with Sheet1 being the active worksheet. The active tab usually has a white background behind the tab name.
Instead of using two-dimensional cells, some spreadsheets are organized by objects, which are variables with a number of attributes attached. This allows the data to be viewed, analyzed and mapped in a number of different ways.