Orbs are pale and circular anomalies, see below, that have been linked with the paranormal.
The orb in this picture is at floor level, middle of photo next to the stack of 4 bluey coloured cases.
Orbs can be of any colour, but are typically off white, can be any shape, but are usually circular. They can appear crisp or fuzzy, in isolation, multiples or clusters.
Orbs, properly, are seen on stills photographs (usually from digital cameras) or on nightshot camcorder footage since the mid 1990s. Various circular light anomalies have been seen with the naked eye, but these have little to do with 'orbs'.
Orbs range in size within a photograph/screen but they tend to appear smaller than 1/10th diameter of the photo frame. Also, orbs tend to be seen in 'haunted place' - hence why people think they are paranormal - but there are very specific reasons for this as can be found on the orbzone.com site.
Orbs on stills cameras and camcorders were not identified and called 'orbs' until the mid-1990s.
What we today know as orbs did exist before this time but were so rare that they did not excite interest in the paranormal research community.
Photographic evidence of orbs has been traced back to the early twentieth century but none were published or linked with the paranormal until 1980 in Rickard and Kelly's Photographs of the Unknown. This book defined the phenomena as unusual but natural. No other examples of pre-mid 1990s orbs being linked with the paranormal have come to prominence as yet.
The effective 'discovery' of orbs took place when, entirely suddenly, paranormal researchers were startled by orbs appearing on dozens of photographs of 'haunted houses'.
This coincided with the mass production and distribution of digital stills cameras.
Orbs became increasingly noted by paranormal researchers in the years that followed.
In 2002 there was an explosion of paranormal investigation groups following the airing of the first season of UK cable TV show Most Haunted.
Most Haunted made the assertion that orbs are the manifestation of spirits, convincing thousands of fans, many of whom went on to become paranormal investigators themselves.
In the years that followed 2002 orbs became derided as investigators saw them too often and the rational theories of orbs gained popularity. These rational theories were unscientifically tested by scores of researchers but no scientific evidence has been established until now.
At the time of writing most paranormal groups appear to be sceptical of orbs: some rule them out wholesale and many rule them out unless they fulfilled certain 'characteristics' (see your questions answered) whilst a few researchers dogmatically cling to the idea that orbs are paranormal.
Many of those who do rule out orbs have a very poor understanding of the theory involved and, thus, make many errors during analysis.
Despite the OfCom ruling that Most Haunted is a program for entertainment and "should not be taken seriously" they have not retracted their assertions about orbs. As such many armchair enthusiasts are still convinced that orbs are paranormal.
According to the orb zone theory, and orb is:
"An out-of-focus dust particle that is photographed within the 'orb zone' of any stills camera or camcorder."
Information supplied by theorbzone.com