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The native horizontal spatial reference for ERA5 data is a T255 reduced Gaussian grid, equivalent to a horizontal resolution of about 31km or 0.3deg.   When you download ERA5 data this is the default horizontal resolution, but optionally you can have the data interpolated to a custom horizontal resolution.

The native file format for ERA-Interim ERA5 is GRIB, with some parameters stored as GRIB1 and others as GRIB2.When but when you download ERA5 data you can specify the output data to be in choose between the native GRIB format or in have the data converted to NetCDF format. This determines your options for the output horizontal resolution:

• If you specify GRIB as output format you will get a GRIB1 or GRIB2 file (depending on the parameterGRIB2 file (for model level parameters) or GRIB1 file (for other parameters, unless otherwise indicated). By default the output will be on the native T255 grid, but you can specify a different Gaussian grid or a lat/long grid:
• If you specify a Gaussian grid other than the default T255 the data will be interpolated to your chosen resolution. The default interpolation method is bilinear for continuous parameters (e.g. Temperature) and nearest neighbour for discrete parameters (eg. Vegetation).
• If you specify a lat/lon grid the data will be interpolated to your chosen resolution. . The The default interpolation method is bilinear for continuous parameters (e.g. Temperature) and nearest neighbour for discrete parameters (eg. Vegetation). The lat/long equivalent of T255 is 0.703125 deg (360/(2*(255+1))). You could use this resolution for parameters stored in GRIB2 format, but GRIB1 format only supports three decimals, so we recommend you in any case round the resolution; we recommend to 0.75x03x0.75 3 deg.
• If you specify NetCDF as output format: Our NetCDF implementation only supports regular grids, so when you extract data it is automatically interpolated from the native Gaussian grid to a regular lat/long grid. The default interpolation method is bilinear for continuous parameters (e.g. Temperature) and nearest neighbour for discrete parameters (eg. Vegetation). Regarding resolution, the lat/long equivalent of T255 is 0.703125 deg (360/(2*(255+1))). You could use this resolution for parameters stored in GRIB2 format, but GRIB1 format only supports three decimals, so we recommend you in any case round the resolution; we recommend to 0.75x03x0.75 3 deg.

For ERA-Interim ERA5 data in all representations the assumed underlying earth model (the geodetic datum) is a sphere with with radius 6367.47km. The surface of the ECMWF model is defined by the orography which indicates the geopotential (from which you can calculate the geopotential height by dividing by 9.80665) of that surface relative to mean sea level. The orography has been interpolated from a higher resolution (DTM/DEM) dataset, GTOPO30, plus some fixes for Antarctica and Greenland, see Chapter 10 Climatological data, of combination of SRTM30 and other elevation datasets (for details see Part IV. Physical processes, of the ERA-Interim IFS model documentation, Chapter 11. The original (~1km) 2.2 Surface elevation data at 30 arc seconds) The SRTM30 orography data is referenced in the horizontal with respect to the WGS84 ellipse (which defines the data major/minor axis) ...

<adapted for ERA5 until here>

but the geodetic lat/lon of the orography dataset are used as if they were the spherical lat/lon of the ECMWF model. In the vertical the data is referenced to the Geoid (EGM96).

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• ERA-Interim is  produced and stored as spectral coefficients with a truncation of T255 or on the N128 reduced Gaussian grid (depending on the parameter), both on a sphere with radius 6367.47 km, as specified in the WMO GRIB Edition 1 specifications.. See ERA-Interim: What is the spatial reference

• ERA-40 has a resolution of T159 (triangular truncation of 159), N80 (80 latitude circles, pole to equator), L60 (model levels), 23 pressure levels and 15 isentropic levels.
• ERA-15 has a resolution of T106 with 31 vertical hybrid levels.

For a list of spectral, gaussian Gaussian and eqivalent equivalent lat/lon grids see the Open IFS FAQ > OpenIFS questions: general and runtime >    "What does the 'T' mean in 'T511', 'T1279' etc?"      and "How do I know the grid from from the 'T' number? 

Further reading:

Berrisford, P, Dee, DP, Poli, P, Brugge, R, Fielding, K, Fuentes, M, Kållberg, PW, Kobayashi, S, Uppala, S, Simmons, A (2011): The ERA-Interim archive Version 2.0. ECMWF, ERA Report Series (specifically section 2)"

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