Acidity - Contrary to how it's used in other industries, acidity actually doesn't refer to the PH level of the coffee. Rather it's how "bright" or "lively" the coffee tastes and is often associated with fruity-flavours. The acidity is closely tied with higher quality coffees - coffees grown at higher elevations grow slower, which allows the "organic acids" to more fully develop.
Body - Used to describe how thick or texturous a coffee is, coffee body can impact how you perceive the flavours. Heavier bodied coffees generally tend to stick around longer on your toungue, and are more pronounced with darker coffees.
Aroma - This one is straight forward: How does the coffee smell after brewing? It's tied the flavour of the coffee itself but can be misleading in some cases. The aroma of coffee can be sweet and intoxicating. Coffees that are fresh roasted (as opposed to store-bought) typically have the strongest aroma, as the coffee degrades within weeks of being roasted. For maximum freshness, buy whole bean coffee and grind just before brewing.
Complexity - Refers to the balance of flavours in a coffee bean and is highly dependent on soil conditions in the originating country. Different balances of nutrients will cause the coffee bean to develop differently and makes some flavours stronger than others.