BNG - British National Grid, the map projection used for Ordnance Survey mapping in Great Britain.
GSS - Government Statistical Service
GSS Code - many of the geographies that we use (wards, local authorities, countries) are assigned 9 character codes by the Government Statistical Service. Hence GSS codes. The GSS code for Sheffield is E08000019.
LSOA - Lower Super Output Areas are a set of polygons that together cover the entire United Kingdom. On average, each one has a population of 1500 people. A lot of census data is produced at the LSOA level.