**Demand**is the relationship between the price of an item and the number of units that will sell at that price. In other words,

Demand →$p(q)$

where p is the price and q is the number of quantity. Usually, $p(q)$ is expressed as the equation

$p = mq+b$

**Revenue**is the amount of income a company makes. The revenue function is expressed as

$R=pq$

When you know what the demand is, then you can express $R$ as a function in terms of $q$.

**Cost**is the amount of money a company needs to produce the items they are selling. It is usually expressed as $C(q)$.

**Profit**is the net amount a company makes. It can be calculated by subtracting revenue from cost. In other words,

$P(q)=R(q)-C(q)$