SNCTOOLS is a collection of MATLAB codes that were written to provide read/write access to netCDF files. It can utilize a few different backends, including MATLAB's native netCDF interface, the old mexnc interface (you only need this if your MATLAB release is 2008a or earlier), and also the netcdf-java package from Unidata, allowing read access to OPeNDAP URLs and netCDF-4, GRIB, and HDF files.
Please note that neither SNCTOOLS nor MEXNC are supported by the MathWorks, so please do not ask the MathWorks for support regarding them. If you have questions about SNCTOOLS or MEXNC, please continue to use the contact email address listed on this site, email@example.com.
MEXNC is a mex-file interface to NetCDF files for MATLAB, and has roughly a one-to-one equivalence with the C API for NetCDF. If you wish to use MEXNC, you need to familiarize yourself with the netCDF C API at Unidata.
MEXNC is now layered on top of MATLAB's netcdf package, so if you have any old code that uses mexnc, it should end up calling the netcdf package "under the hood". However, if you need that level of control over writing netcdf files, you should probably look to rewrite your code to not use mexnc anymore and just call the netcdf package instead. SNCTOOLS does just that...