Using empty blocks
In addition to providing blocks pre-built with all sorts of elements, GroovePages also provided "empty" blocks. These empty blocks (found under the "Empty" category in the blocks sidebar) can be used to create custom blocks.
Under the "Empty" category, you will find "layout" blocks (typically one or more columns) and an empty "container" block.
In the screenshot above, the green arrow indicates the empty container block and the blue arrows indicate layout blocks. The "empty container" block differs from the layout blocks in the sense that it does not yet contain a pre-built layout (you can actual grab layout elements and place these into the empty container block). The layout blocks are simply (empty) containers blocks + a layout element.
If you're looking to create a block with content that spans the full width of the page, you're probably best of using the empty container block. If you're to create a block with a grid layout, the layout blocks make the most sense.
Difference between empty container block and the one column layout block
The empty container block and the one column layout block look very similar; and they are. The empty container block does not contain a layout element, and the layout blocks do. If you're planning to create a layout using layout elements, you'll want to start off with the empty container block. If you're looking for a simple grid layout or a single column that spans the full with the page, you're best off using one of the layout blocks.