a real world point of view
Foley is that sound in a multimedia production which is neither dialogue nor musical score. Persons who perform foley are called "foley artists". The process of foley recording is generally similar to that for ADR; foley artists will watch a scene play out, then synchronise their sound with the action on screen. The craft demands creativity from its practitioners. Foley artists must find and use objects so their sounds unobtrusively "reads" true to the scene. For instance, the sound of David Tennant tossing and catching his sonic screwdriver is achieved with a butterfly wine opener, yet is likely believed by the casual viewer to be the actual sound the prop makes when tossed around.
Foley pre-dates both television and "talking" motion pictures. Foley sound probably debuted in live theatre, in which specific sounds, like a doorbell, would occasionally be required. Many live theatre productions would not have necessarily required a dedicated technician, but could instead have been "performed" by a stagehand. It came to be a proper "art form" with dedicated technicians during the era of live radio drama. Nevertheless, the name itself derives from Jack Donovan Foley, a sound editor at Universal Studios. The role of the foley artist has increased in direct relation to the ability to record and layer multiple sounds into a single soundtrack. In the 21st century, most film and television productions tend to foley virtually every possible sound, from footsteps to the clink of a fork on a dinner table.
During most of the classic era of Doctor Who, the number of tracks that could be layered into the completed soundtrack was limited. Detailed foley work was impossible and was generally limited to futuristic objects, like the TARDIS, which absolutely required a distinctive sound. Some would argue this is not truly foley work — but rather sound design —as such "special" sounds are not performed and captured live. Rather, sounds like that of the TARDIS dematerialising originate with ordinary objects, are heavily treated and edited and saved for use as needed by the sound editor. Seen in this light, it may be correct to say that foley work in Doctor Who is limited to the McGann movie and the 2005 series.
In the BBC Wales production of the show, each episode is meticulously combed for foley opportunities. Even minor sounds, such as the rustling of jewelry as an actor turns, is recorded on a special foley track. This is layered into the final soundtrack. Doctor Who now has at least two foley artists on its payrolls, Julie Ankerson and John Fewell. They, along with a foley studio recording session, were featured in the tenth episode of the fourth series of Doctor Who Confidential in an episode entitled, "Look Who's Talking".
This episode also explained that the foley studio session was saved as its own track so it could easily be temporarily removed. This makes it easier for foreign broadcasters to record a dialogue-only track, then nest the new recording back into the overall soundtrack. This reduces the time needed to dub the episode into another language, yet retains the integrity of the overall soundtrack.