To make things easy, we often speak about left bank and right bank wines of Bordeaux. By 'banks' we refer to the areas located on one side or the other of the River Garonne that divides the Bordeaux wine region into two distinct areas.
To the south of the River Garonne is where the Left Bank is located and the most famous wine regions include the Médoc, Pessac-Léognan and Graves. To the north of the River Garonne the most famous wine producing appellations include Saint Emilion, Pomerol, Fronsac or Blaye.
Due to a difference in the type of soils that can be found in each region, the wines produced in each area are also very different. On the left bank, the soil is rockier, made up of gravel and deep limestone whereas on the right bank the soil is made up of pure limestone sometimes mixed with sand or clay.
Because the soils are different, the main grape varieties used in the blend are also different since the choice of the dominant grape variety depends on the type of soil that can be found on an estate. Remember, in France it not permitted to import some grapes from another wine producing region which means each property must grow the grapes needed to make the wine on-site. In general (there are always exceptions in France) left bank wines will use Cabernet Sauvignon as the main grape variety of the blend while right bank wines will use Merlot.
As such, left bank wines tend to be more full bodied type of wines, higher in tannin and that require a certain ageing so that these tannins become smoother with time. On the other hand, right bank wines are generally more elegant and fruity type wines that have less powerful tannins.