Multiple sclerosis (MS) is a neurological condition; it is a condition of the Central Nervous System (CNS). It is also an auto-immune disease. These means that the body's own immune system attacks itself (instead of invading foreign bodies, such as viruses).
In MS the fatty coating around the nerve fibre (myelin) is damaged - which causes a wide range of neurological symptoms. Over time this de-myelination can cause plaques or scarring, and even damage the nerve itself (which can lead to permanent disabilities). Think of it like electrical wiring with the insulation worn away and think of all those sparks flying off in the wrong directions!
Because the Central Nervous System pretty much runs the body that means that symptoms can affect pretty much any part of the body in a variety of ways depending on which part of the nerve is effected.
The cause of MS is not known. It is suspected that there is a mix of environmental and genetic factors; perhaps a susceptible gene which is triggered by an external factor (causes from lack of Vitamin D to viruses are being investigated). What is known is that it becomes more common the further from the equator you travel and is almost twice as common in women than in men.
MS comes in many forms: the relapsing/remitting form is characterised by periods that are relatively symptom free interspersed with 'attacks' (relapses) where symptoms present. The primary progressive form is a debilitating form where the symptoms continue to worsen and become more disabling as time passes with no remission. Secondary progressive can occur after many years of relapsing/remitting when the periods of relapse diminish or vanish. People also may describe their MS as 'benign' (very few relapses, or relapses that are not very disabling) or 'aggressive' (many relapses or relapses that are very disabling).
MS has no cure. There are many forms of disease modifying therapy available which can decrease the severity/length/frequency of relapses. There is also a plethora of drugs available to treat the symptoms of relapses.
MS is not a terminal disease. Not everyone who has MS will end up in a wheelchair. Everyone's experience of MS is different.