mul·ti·task [muhl-tee-task, -tahsk, muhl-tahy-]
verb (used without object)
1. Computers. (of a single CPU) to execute two or more jobs concurrently.
2. (of one person) to perform two or more tasks simultaneously.
Do we really multitask?
This answer from About.com on the article Can People Really Multitask? sums it up well:
The short answer to whether people can really multitask is no. Multitasking is a myth. The human brain can not perform two tasks that require high level brain function at once. Low level functions like breathing and pumping blood aren't considered in multitasking, only tasks you have to "think" about. What actually happens when you think you are multitasking is that you are rapidly switching between tasks.
The cerebral cortex handles the brain's "executive controls". Those are the controls the that organize the brains tasks processing. The controls are divided into two stages.
The first is goal shifting. Goal shifting happens when you switch your focus from one task to another.
The second stage is rule activation. Rule activation turns off the rules (how the brain completes a given task) for the previous task and turns on the rules for the new task.
So when you think you are multitasking you are actually switching your goals and turning the respective rules on and off in rapid succession. The switches are fast (tenths of a second) so you may not notice them, but those delays and the loss of focus can add up.
According to this article on NPR - with Clifford Nass, The Myth of Multitasking:
NASS: The research is almost unanimous, which is very rare in social science, and it says that people who chronically multitask show an enormous range of deficits. They're basically terrible at all sorts of cognitive tasks, including multitasking. So...Another article, The Myth of Multitasking by Christine Rosen:
In one of the many letters he wrote to his son in the 1740s, Lord Chesterfield offered the following advice: “There is time enough for everything in the course of the day, if you do but one thing at once, but there is not time enough in the year, if you will do two things at a time.” To Chesterfield, singular focus was not merely a practical way to structure one’s time; it was a mark of intelligence. “This steady and undissipated attention to one object, is a sure mark of a superior genius; as hurry, bustle, and agitation, are the never-failing symptoms of a weak and frivolous mind.”Even Forbes weighed in on the topic, as well as this article: Think you can multitask? Congratulations, you're probably living a lie.
I particularly liked this article, Don’t Multitask: Your Brain Will Thank You - which quotes Clifford Nass.