A serial killer is usually defined as a person who kills 3 or more people in 3 separate events with a "cooling off" period between the killings. The FBI profilers tend to disregard the "3 or more people" for good reasons. Indeed, some killers are caught after their first or second killing, but have characteristics that would indicate that they have the potential of killing more people (see Southeast Asia's Serial Killers part two). But in this definition, the separate events and the "cooling off" period are essential to consider a series of murders as serial killings. Often, but not always, there is a deep sexual element to the series. This element can be as obvious as the rape of the victim, or at the contrary subtle such as the sexual arousal of the offender in some other practices (sadism, necrophilia...).
Mass murderers are often depicted as "human time bombs". Rightly so, they are often people who literally see their lives completely fall apart (divorce or separation, loss of a job, humiliation or all the above) and are in a sense pushed to their limits. Humiliation and frustration in this case would lead to an overwhelming rage and the mass murderer would "explode in a burst of devastating violence that wipes out everyone within rage" (Harold Schlechter, The Serial Killer Files). Having lost total control over their lives, they start blaming everything and everyone for their failure, and decide to take down as many people as possible with them as a revenge to the harm that has been caused to him. Mass murderers are almost always commit suicide, either killing themselves, or committing "suicide by cop".
A spree killer kills 2 or more people in a short period of time, in different locations. As the mass murderer, he has suffered a major loss or a major humiliation, and is so enraged that he is pushed over the limit. The major motivators of spree killers are revenge against society or a desire to prove everyone that they should not be messed with.