Attention is an umbrella term that describes a variety of behaviors and functions. Michael Posner identified three discrete subsystems that, together, comprise what we generally regard as attention. These three components are: alerting, orienting, and executive control.
Alerting is the ability and preparedness to attend to (and to sustain attention to) high priority stimuli. Orienting is the selection of attention to high priority stimuli. Executive control of attention is the ability to resolve competition for attention (e.g., ignoring an irrelevant stimulus).
In 2002, Posner and colleagues developed the Attention Network Task (ANT), a behavioral task that includes measures for all three subsystems.
Fan, J., McCandliss, B. D., Sommer, T., Raz, A., & Posner, M. I. (2002). Testing the efficiency and independence of attentional networks. Journal of Cognitive Neuroscience, 14(3), 340-7. doi:10.1162/089892902317361886
Posner, M. I., & Petersen, S. E. (1990). The attention systems of the human brain. Annual Review of Neuroscience, 13, 25- 42