Anxiety is defined by the APA (American Psychological Association) as "an emotion characterized by feelings of tension, worries thoughts, and physical changes like increased blood pressure." Anxiety itself is a very normal, and often healthy emotion - but when someone is feeling a disproportionate levels of anxiety that it may have become a disorder.
Feelings of anxiety will pop up when a person is facing potentially harmful situations or triggers, and are not only normal, but necessary for survival! Since the beginning stages of humanity, predators nearby and incoming danger will automatically set off alarms in the body that allows you to utilize evasive action. These alarms are most noted by physical changes such as elevated heartbeat, increased sensitivity to your surroundings, and sweating.
The threat of danger causes an adrenaline rush, which in turn triggers a person's "fight or flight" response. This is the moment that prepares humans to physically confront their attacker, or flee.
In the times when we were running from enormous predatory animals and working to escape their threats, anxiety seems to make sense. Purely evolutional. However, these days, we aren't trying to escape a beast four times our size - we now struggle with things that are more mentally challenging beasts, such as work, health, family life, and money. Our anxieties now do not cause a fight or flight sensation, but still affect us just as dramatically.
Anxiety Disorders
The length of time spent revolving around your anxious emotions can sometimes be out of proportion to the original stressor. When you have an over-abundance of anxiety, this can also affect your physical wellbeing, causing problems with blood pressure and nausea. Once you've hit this point of more anxiety than naturally necessary, you're knocking on the door to an anxiety disorder - defined by the APA as "someone having recurring intrusive thoughts or concerns." Anxiety at this stage can absolute affect your day to day functioning.