I downloaded ERA-Interim data. Where can I find the exact definitions and units of measurement for the parameters (variables)?
Please look up parameter details in the ECMWF parameter database, http://apps.ecmwf.int/codes/grib/param-db. Search on the top right.
The database lists all parameters used at ECMWF with their Name, Short Name (not unique!), Unit, Parameter ID (unique), and available file formats.
The database includes all parameters used at ECMWF, which are many more that what is available for ERA-Interim.
ERA-Interim data is available primarily in GRIB format. A single file can contain multiple parameters, for example temperature, precipitation, evaporation, and so on. In the file each parameter is identified by the parameter’s name ('Evaporation'), a unique short name ('e'), and a unique parameter ID ('182').
If you already have a GRIB file and only want to see what parameter it includes you can use the tools from ECMWF's ecCodes tools (grib_ls, grib_dump), the CDO info operators (info, infon, sinfo, sinfon) or as a graphical tool you can use Panoply. For further options see How to read or decode a GRIB file.
While you can download data from ECMWF in NetCDF format, the NetCDF format is unsupported and files downloaded in NetCDF format do not necessarily contain all selected parameters and all metadata.
For a general documentation of ERA-Interim please see the ERA-Interim archive document: http://www.ecmwf.int/en/elibrary/8174-era-interim-archive-version-20
ERA-Interim production to stop on 31st August 2019
As ERA5 is now available (What are the changes from ERA-Interim to ERA5?), we are preparing to stop the production of ERA-Interim on 31st August 2019. This means that the complete span of ERA-Interim data will be from 1st January 1979 to 31st August 2019.
Keeping in mind that ERA-Interim is published with an offset of about three months from the dataset's reference date, ERA-Interim August 2019 data will be made available towards the end of 2019.
For the time being and until further notice, ERA-Interim shall continue to be accessible through the ECMWF Web API. ERA5 is available from the Climate Data Store (CDS).