Psychosis describes a state where the sufferer, in some way, loses a degree of touch with reality. This tends to express itself in the thoughts or experiences of the individuals.
Typically, this may take the form of experiences whereby the individual has the experience of percieving a sound (eg. a voice) or a sight (or sometimes a touch or smell) where, in actual fact, no such sound, smell, touch etc took place. Such an experience is known as a hallucination (eg. an auditory hallucination = of sound; visual hallucination = of sight; somatic hallucination = of touch; olfactory hallucination = of smell).
Another typical feature of psychosis is the experience of unusual thoughts or beliefs that are usually untrue - although the individual believes them with total and unshakeable conviction (even in the face of what seems to every else to be clear evidence to the contrary). Such thoughts and beliefs are known as delusions. A common form of delusion is the belief that 'they are out to get me' (whether they refers to aliens, neighbours, the police, MI5, terrorists etc) - these are termed 'persecutory' delusions.
In addition to hallucinations and delusions, other features of psychosis may occur, such as disorders of the patterns of thinking (which often becomes obvious in a rather muddled or illogical manner of speech) - this is termed 'thought disorder'. Although schizophrenia is the most common illness that causes psychosis, it can occur in a wide range of other conditions, including bipolar affective disorder (sometimes known as 'manic depression'), severe depression, infections, states of extreme stress or anxiety, or as the consequence of the abuse of drugs or alcohol.