First of all, what is stress? Stress is a state of excitement. Excitement in terms of the human body means to increase. Up the HR (Heart rate)? You excite HR. Take the HR down? You depress HR. There are two types of stress. Eustress and distress. Eustress is good stress. This stress is, for example, what makes you stronger and faster. Go bench press? You stress your body and it gets stronger. Distress is stress you want to avoid. It can tear your body down, not build it up. The emotions excitement and depression are a different thing.
Simply put, anxiety is a low key form of fear, sometimes a high key form. The adrenal Glands (Also what dumps adrenaline into your system) pump out a hormone called Cortisol. Cortisol is considered a "stress hormone". It causes distress, not eustress. As the name suggests, it stresses the body primarily getting kick-started itself through fear or anger. This hormone helps excite your body and kick-start your SNS (Sympathetic nervous system, or 'fight or flight') to dump adrenaline, endorphins and various hormones into your system to prepare for some shit to go down. For example, it is meant to be used when you run into a situation where you have to fight for your life, or run like your life depends on it. Ran into a bear? This is where you want your SNS to click on. The more you excite the body the stronger the SNS kicks in. Anxiety is the fight or flight system kicking in, but in a small way. If it kicks in in a big way, but within modern living, you can get panic attacks. You can hear stories of people hearing a gunshot, SNS clicking on, running like the wind, and 10 minutes later, when they get somewhere safe, realize they got shot through the shoulder. Not cause they felt it, but cause something felt 'wet' (blood). That's what it is meant to be useful for.
It is a positive thing for us to have, but can turn into a very negative thing if you are in a constant state of excitement. Cortisol destroys the circulatory system if it's a constant state.
What does anxiety feel like? You get nervous, jittery, stressed and 'flighty'. If it clicks on in a big way, it's like being on methamphetamine with a high dose of morphine. You don't feel shit and you are AMPED in a big way. But you can't run, you can't fight, you have to sit there in a state of excitement, and this can lead to panic attacks.
You've had anxiety before, I guarantee it. If you have not, you may need to get checked out. Ever get that butterfly feeling asking a girl out? Ever get that nervous feeling before giving a speech in front of a crowd? Or been in a situation you were not sure about, iffy about? Taken a test an not sure how you did? Not sure if you'll get this homework done in time?