A galette is a flat tart, sweet or savoury, where the pastry is not baked in a specific pan or dish but cut into any shape you want. Rectangles and squares are popular. They can be very free-form or more precise in shape, as in the image.
To create a border, when using puff pastry, use a sharp knife to draw a line about 2 cm in from all the edges, leaving a nice shallow cut. This will puff up and create a fantastic edge. When using shortcrust pastry the edges are often rolled or pinched around the filling to create a raised, decorative edge.
One of my standard savoury galetes is 10cm squares of puff pastry with 2 cm border, topped inside the border with pesto, just a couple of fresh tomato slices and some marinated fetta; baked till golden, and served as a snack. They make great individual tarts. (Go to Careme Pastry for some excellent hints.)