Asthma is one of the most known illnesses that are seen as chronic (meaning ongoing and usually livelong) without posing severe threat to life. Asthma is a respiratory - lung - based illness, effectively caused by inflammation of the linings of the tubes in the lungs. This, in turn, means the tubes (the medical term is the "bronchi" are narrowed, making it harder for sufferers to breathe comfortably.
Asthma tends to manifest itself in two different ways; a constant problem with occasional 'flares' - known as asthma attacks. The most common form of treatment is inhalation based medication, which is used every day to prevent the bronchi from narrowing and causing discomfort.
As asthma is a respiratory disease, asthma symptoms are all based around the lungs and breathing. The most common asthma symptom is shortness of breath for no apparent reason - particularly when the body is 'at rest', i.e. when you are sitting down and not being physically active. This is usually coupled with a feeling of tightness in the chest, which is a direct result of the aforementioned narrowing of the bronchi, which in turn restricts air flow.
This restricted air flow means that sufferers may also wheeze when suffering an attack; a very definite sound caused by poor breathing. Coughing is also prevalent amongst asthma sufferers, and is the most common asthma symptom outside of an attack; most sufferers will always be prone to coughing fits, due to the inflammation of the bronchi.
Diagnosing asthma is relatively simple, so if you have any of the above symptoms and are concerned, see your doctor.