Demand in economics

Supply and Demand Curves
At any given point in time, the supply of a good brought to market is fixed. In other words, the supply curve, in this case, is a vertical line, while the demand curve is always downward sloping due to the law of diminishing marginal utility. Sellers can charge no more than the market will bear based on consumer demand at that point in time.
Over longer intervals of time, however, suppliers can increase or decrease the quantity they supply to the market based on the price they expect to charge. So over time, the supply curve slopes upward; the more suppliers expect to charge, the more they will be willing to produce and bring to market.
For all periods, the demand curve slopes downward because of the law of diminishing marginal utility. The first unit of a good that any buyer demands will always be put to that buyer's highest valued use. For each additional unit, the buyer will use it (or plan to use it) for a successively lower-valued use.
Shifts vs. Movement
For economics, the "movements" and "shifts" in relation to the supply and demand curves represent very different market phenomena.
A movement refers to a change along a curve. On the demand curve, a movement denotes a change in both price and quantity demanded from one point to another on the curve. The movement implies that the demand relationship remains consistent. Therefore, a movement along the demand curve will occur when the price of the good changes and the quantity demanded changes per the original demand relationship. In other words, a movement occurs when a change in the quantity demanded is caused only by a change in price and vice versa.
Like a movement along the demand curve, the supply curve means that the supply relationship remains consistent. Therefore, a movement along the supply curve will occur when the price of the good changes and the quantity supplied changes by the original supply relationship. In other words, a movement occurs when a change in quantity supplied is caused only by a change in price and vice versa.
Shifts
Meanwhile, a shift in a demand or supply curve occurs when a good's quantity demanded or supplied changes even though the price remains the same. For instance, if the price for a bottle of beer was $2 and the quantity of beer demanded increased from Q1 to Q2, there would be a shift in the demand for beer. Shifts in the demand curve imply that the original demand relationship has changed, meaning that quantity demand is affected by a factor other than price. A change in the demand relationship would occur if, for instance, beer suddenly became the only type of alcohol available for consumption.
Conversely, if the price for a bottle of beer was $2 and the quantity supplied decreased from Q1 to Q2, there would be a shift in the supply of beer. Like a shift in the demand curve, a shift in the supply curve implies that the original supply curve has changed, meaning that the quantity supplied is impacted by a factor other than price. A shift in the supply curve would occur if, for instance, a natural disaster caused a mass shortage of hops; beer manufacturers would be forced to supply less beer for the same price.