Attention in its colloquial sense is conscious concentration on something, such as listening carefully to what someone is saying, or noticing something. Generally a person can only attend to a few things at a time and even trying to attend to more than one thing at a time, such as driving and talking on a cell phone, may result in reduced efficiency.
On the other hand, psychology and cognitive science had to come up with a different definition and a different view on attention: Picture yourself reading a book. What are you doing? Your eyes are jumping from word to word and from line to line, your visual attention thus must be directed towards the words and letters on the page. But if you are reading a good book, your conscious mind is not concentrating on letters. It is concentrated on the story, the fate of some character in the novel. Good books are easy and fun to concentrate on. Picture yourself reading a really good novel. You are deeply immersed in the story you are reading and suddenly your phone rings. Again, your attentional system is required: while it was helping you to concentrate by filtering out the noises coming through the window, the sensations from your body, maybe even the growing hunger you would otherwise have felt just moments ago, it now has to shift from the book to the phone.
It thus becomes clear that there are several levels and areas of attention that have to be distinguished when studying attention scientifically: Visual and auditory attention, top-down and bottom-up controlled attention, and so on.
Scientific models of attention often include aspects of selectivity, selecting one item in favor of another. Either the selected one is enhanced, or the other one is suppressed. Other models discuss the assignment of resources to items. Due to the resulting reduction of data to be processed, models of computer vision sometimes use mechanisms of visual attention. Another aspect of attention is that it allows to select relevant items and suppress distractors for task specification. Image a lion hunting a herd of gnus. Having all the positions and directions in its mind together makes no sense. Just by selecting one of them allows to specify the necessary movement to catch it.
