These terms mean different things:
(http://www.politics.co.uk/reference/green-belt) – uses some quotes from the NPPF
A Green Belt is an area of land protected from development. Green Belt land surrounds cities and towns to inhibit ‘urban sprawl’, prevent neighbouring towns merging into one another, preserve the countryside and the setting and character of historic towns, and assist in urban regeneration by promoting ‘brownfield’ development (development on derelict urban land).
Green Belt land is intended to be kept permanently open.
Also land can be allocated (or returned to) Greenbelt status, even Brownfield – obviously certain criteria have to be met.
Greenbelt is outside the urban area.
Here is a link to a map of Greenbelt areas – http://www.telegraph.co.uk/earth/greenpolitics/planning/9708387/Interactive-map-Englands-green-belt.html
Are sites which have not previously been built on. This includes the greenbelt land around cities.
There are various definitions – here is the NPPF
The Local Green Space designation will not be appropriate for most green areas or open space. The designation should only be used:
- Where the green space is in reasonably close proximity to the community it serves;
- Where the green area is demonstrably special to a local community and holds a particular local significance, for example because of its beauty, historic significance, recreational value (including as a playing field), tranquillity or richness of its wildlife; and
- Where the green area concerned is local in character and is not an extensive tract of land.
Green Space is essentially within an urban area (or on the fringe) and has some protection from building by statute (I think) whereas Greenfield is outside the urban area without the protection of Greenbelt status. Most of Britain is Greenfield, there being only 14 Greenbelt areas.