The gi_taxid_nucl.dmp is about 160 MB and contains  two columns: the nucleotide's gi and taxid.
The gi_taxid_prot.dmp is about 17 MB and contains two columns:  the protein's gi  and taxid.

The file gi_taxid_nucl_diff.dmp contains differences between the latest and previous 
dump files of the nucleotide's gi  and taxid;
The file gi_taxid_prot_diff.dmp contains differences between the latest and previous 
dump files of the protein's gi  and taxid;

The first (left) column is the GenBank identifier (gi),
the second (right) column is taxonomy identifier (taxid).
The files are updated every Monday around 2am EST.

Please do not attempt to view the files with web browser (some of them could be huge). 
You can download the file with netscape by right mouse click and 
select 'Save link as...' or use ftp instead.