Emboli are most often blood clots or pieces of plaque that have broken off from large arteries that lead to the brain. Emboli float in the blood stream until a vessel is blocked. The blockage may cause a lack of oxygen in that area of the brain. The blockage may also cause the vessel to break open, causing blood to pour into an area of the brain.
The most common place for blood clots to form are in the heart. The treatment for strokes caused by cardio-embolic clots is generally anti-coagulation, a blood thinner. Strokes caused by pieces of plaque are generally treated by anti-platelets and statins.
A wonderful resource for learning more about stroke is stroke.org.