Grid Setup Window
On this window you can configure the grid and background settings. Click the grid icon in the statusbar or select Grid Setup from the Edit menu to access this window. The grid helps aligning your map elements and the 64 mappixels grid is important to Doom as it indicates how flats are aligned. You can change the size of the grid you want to work with at the top of this window. Note that the grid can also easily be changed with the actions Grid Increase and Grid Decrease.
The background is useful when drawing over an image to copy its shape. Check the Show background image checkbox and select a texture, flat or file to be used as the background image. You can also change the offset and the scale.