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