All animals which are capable of fighting and/or fleeing have a "fight or flight" impulse which is triggered when they feel severely threatened or frightened. Human beings are not exempt from this basic biological survival impulse. When threatened, we have to choose, often without conscious thought, whether to stay and fight or flee. The fight-or-flight impulse includes an increase in heart rate, a tensing of muscles, increase in awareness, and a rush of adrenaline. This gives you the boost you need to fight or run for your life.
However, in some people, this fight-or-flight reaction becomes exaggerated and may be triggered by what may be very minor stimuli, or occasionally by nothing at all (at least, nothing the person can logically determine). The generic term for this is anxiety, and it is classified as a disorder when the symptoms occur regularly and often.
Anxiety Disorder is the catch-all umbrella term for a whole host of other, more specific ailments such as agoraphobia and panic disorder. Related are Post Traumatic Stress Disorder, eating disorders, and more.
A panic attack is generally a violent, very intrusive experience which reaches peak intensity within about a minute. An anxiety attack is much less "intense", but can last for some time, sort of a continual, lower-level state of perpetual anxiety, always perched to fight or flee. Do note that the terms "anxiety" and "panic" are used somewhat loosely by some people, and may have more or less the same meaning, or may be used to designate different things. This is how I understand this, and how I have experienced it, and these are all rather closely related disorders, anyway.
Symptoms of Generalized Anxiety Disorder:
Symptoms of a Panic Attack
Obviously, these symptoms can make life extremely difficult, even painful. It can become impossible to function normally, such as being unable to leave your own home (agoraphobia) or the problems which are associated with avoidance, where the sufferer will simply seek to utterly avoid any situation which is likely to trigger symptoms. Also, since panic attacks can happen sometimes for no apparent reason, they can disrupt a sufferer's life considerably.
Sometimes people make light of the term "panic attack" or think it's not serious enough to worry about, some sort of weird neurotic problem, or some kind of personal weakness (you know, just "Pull yourself together!"). However, the terror and agony of a panic attack can be very serious. The attacks are not literally damaging, but the after-effects can be difficult to get past, and while having an attack, a person may well be genuinely unable to function until it passes.
Anxiety and stress do have long-term, ill effects on a person's body and soul. Anxiety can cause hair loss, raise the risk of heart attack, memory loss and poor work or school performance, irritability, and is associated with chronic digestion problems including ulcers, and some forms of chronic pain (headaches, for example).
As for what causes anxiety disorder, the answer is complex. Like most psychological disorders, it appears to have its root in biology, and can be effectively treated with medication. However, there is also a very close tie to the same causes as those suffering from chronic dissociation and/or PTSD (which is, after all, classified as an anxiety disorder).
My personal reflection is that when one is required to live in a continual fight-or-flight mode for a long time, it becomes a permanent state of existance, and when exposed to a lot of stress for a long time, you just get in the biological and physiological habit of being anxious.
I have suffered from anxiety disorder for years. I was, for nearly a year, so anxious I couldn't leave my own apartment alone because the panic symptoms overwhelmed me. It was a classic case of agoraphobia. As that eased, I discovered the meaning of "panic attack". I had, in fact, had them for years, but had no idea what they were. In my case, a panic attack includes a heart that pounds like a frightened rabbit, a feeling of being choked and unable to breathe, severe dizziness, disorientation, a feeling of "going crazy", and a very strong urge to run and hide. I am fairly certain that I have also taken the "fight" option of fight-or-flight, and probably provoked plenty of unpleasant encounters that way.
I don't have major panic attacks often any more. I have minor ones from time to time, but where they used to happen several times a week and were so severe they left me utterly exhausted and with what I described as a "brain wipe" (I'd forget whatever I was doing, lose all concentration, sometimes losing track of where I was going or why), they now happen no more than a couple of times a month, usually far less often, and are far less intense.
I also sometimes have short-term but intense anxiety attacks, usually if I'm running low on sleep for a long time or if I've overdosed on caffeine. As long as I take care of myself, it's not generally a problem.
I have not been treated with medication for my disorder, because when it was most severe I did not seek medical attention (partly the agoraphobia and partly the stress and pressure of being in an abusive relationship). By the time I did get proper professional attention, I was on medication for mood swings and the panic attacks were becoming less of a problem (the less stress I was under, the less frequent the attacks). I have, however, undergone a lot of counseling and training in relaxtion and coping techniques which were used to treat pretty much all my symptoms, from the PTSD flashbacks to the panic attacks to the overall anxiety.
I am to the point where I can recognize what is likely to trigger an attack and I can sometimes prevent the attack by simply being prepared. Other times, I recognize the pre-attack signs and can prevent the panic attack from happening, but it takes some concentration (and lots of practice). Sometimes a panic attack will still hit me without warning, but usually I can say, "I'm having a panic attack. I feel terrible, and that's okay, but it's just a physical sensation, and I am not in any actual danger." I don't have to react to the symptoms. I don't have to fight or flee. I can stay put and breathe normally and distract myself from the physical symptoms enough to get through it relatively calmly.
As a final note, the symptoms which are associated with anxiety and panic disorder can also be symptoms of other serious medical concerns. A full medical exam is always indicated if you have symptoms which fit this profile. If nothing else, you can get a clean bill of health and eliminate the physical as a possible cause, and so better concentrate on treating the anxiety.