Your boat goes forward depending on the wind it receives.
On a sailboat, the point of sail corresponds to the direction in which the boat advances depending on the wind. Since we cannot control the direction of the wind, we need to adapt the placement of the sails depending if the wind comes from the sides, from behind or from the front.
The points of sail
The wind comes from the front of the boat: upwind
The upwind is the point of sail in which the wind is the closest to the boat while it goes forward. The wind comes from the front side of the boat and the angle between the wind and the boat is around 45°. If this angle decreases even more, the boat will not be able to advance anymore, since the wind will be too directly facing it.
The wind comes from the side of the boat : beam reach
This is the fastest point of sail. The sailboat is perpendicular to the wind. It is the limit between close-hauled and running downwind.
The wind comes from behind the boat : running downwind
The wind comes from behind the boat.
The other points of sail:
There are other terms to describe those points: in irons (when the boat is as close as it can get to the wind), the broad reach, the beam reach and close-hauled (upwind point where the boat does not approach the wind too much).