Close to the rivers of Bordeaux, there is a band of alluvial soil, which is much too rich in nutrients to produce great wine. Only the lowest appellations are grown here. The best wine from bordeaux is grown on gravel, overlying a subsoil of marl, which allows water to drain through, giving the vines that struggle for survival essential for making good wine. Most of this soil is found in the villages of the Haut-Médoc in the north, and in nothern Graves. In Saint-Emilion, the gravel instead covers a base of limestone. In Bordeaux, there are very few hillside vineyards but when they do occur, the soil is usually limestone and clay.
The aspect and slope of a where a vine is planted can also have a large effect on the grapes. A 30° slope facing towards the equator can double the amount of sun the grapes receive. In a flat planting, the vines on either side of any given row will block sunlight to the rows next to it. But when the vines are on a hill, the sun can hit all the grapes hanging down on every row. A sloped hill also offers better drainage, and more air circulation.
The hill also complicates the viticultural proces, because it is harder and therefore more expensive to work on a hill. It often makes mechanical harvesting impossible. In Burgundy, for example, a Grand Cru wine can be grown right next to a Cru Bourgeois, but the Grand Cru will all come from a hill, while the Cru Bourgeois will be on the flat plain right next to the hill.
The kind of soil grapes are grown on has a huge effect on how the wine will turn out. It is generally accepted that a soil that puts stress on the vine makes a better wine. If grape vines had the ideal conditions with plenty of water and nutrient rich soil, they'd spread far and wide, and produce lots of grapes. But that doesn't make very good wine. When vines are stressed, they produce fewer grapes, andy those grapes are much stronger in flavor. This is because the vines are activating a natural defense system. The vine thinks it is dying, so it pours all its energy into making nutrient(and flavor)-rich grapes so it can propagate itself. A stressed vine will also be less leafy, which allows more sunlight to reach the grapes.
In addition, when the soil has plenty of water, the roots spread horizontally to soak up as much water as it can. When the soil doesn't have very much water, the roots are forced to forage deep into the ground, which increases the transfer of minerals, and results in a healthier vine.
Terroir is a very important concept in winemaking. Terrior is how the combination of climate, weather, soil, and topography come together to make a wine different from other wines. Basically, terroir is how all the natural aspects come together to make a wine what it is.
Terroir is much more important as a concept in places like France, where the winemakers know their land and climate very well. This is part of why French labeling is so confusing. Rather than labeling wines varietally, like in California, wines are labeled based on where they are from, which is an indication of the terroir involved in making the wine. If winemaking is done right, subtle aspects of a vineyard, such as the differing sloped or hilled areas, will be represented in the wine, and give that wine its own regional flavor. The wine that is grown on the slope will probably cost a little bit more, and be a little bit more complex than the wine that grows in a flat surface, and French winemakers have had a long time to figure out which areas of their vineyard produce those more complex wines.