An oar is an implement used for water-borne propulsion. Oars have a flat blade at one end. Rowers grasp the oar at the other end.
The difference between oars and paddles is that oars are used exclusively for rowing. In rowing the oar is connected to the vessel by means of a pivot point for the oar, either an oarlock, or a thole. The oar is placed in the pivot point with a short portion inside the vessel, and a much larger portion outside. The rower pulls on the short end of the oar, while the long end is in the water. By contrast, paddles are held in both hands by the paddler, and are not attached to the vessel.
Rowers generally face the stern of the vessel, reach towards the stern, and insert the blade of their oar in the water. As they lean back, towards the vessel's bow, the blade of their oars pivots in the oarlock, and the end in the water moves towards the stern, providing forward thrust.
For thousands of years vessels were powered either by sails, or by the mechanical work of rowers, or by paddlers. It is common for an oar propelled vessel to also have the option to be powered by sail, both in antiquity (for instance the galley) and more recently.