The octagon is a fairly minor shape variant of the Rubik's cube. It is sometimes referred to as the 'barrel'. It is shaped like a barrel - the edges and corners are triangular, the centres are as they are on a standard cube.
You solve the puzzle like a cube. Unlike the standard cube, there is nothing to tell you where the corners go and the edges of the middle layer are not sensitive to orientation (you can't tell if they are flipped). This leads to a situation where you appear to need to swap exactly two pieces or need to flip a single edge. If you don't notice that effect, you may find yourself in a situation where you see a pattern you don't recognize or you can't seem to orient or position the pieces of the last layer. The answer is to swap over the angled columns, thereby changing the position of the corners.