Trajectory File

From wiki.gromacs.org

Trajectory files are files written initially by mdrun to contain atomic positions, velocities, and/or forces. The type of data and the period with which they are written are controlled with .mdp file options (see Using Trajectory Information). GROMACS will write full-precision portable-binary data in a format known as a .trr file and, optionally, a reduced-precision format for positions only, known as an .xtc file. The latter is useful when you know you will want positions for analysis with a higher frequency than the velocities, or only a subset of positions for analysis.

You will need a full-precision frame with both positions and velocities in a .trr file (and maybe energies) for doing a restart, and GROMACS will always write one of these at the end of a run, regardless of the .mdp file options you chose.

The GROMACS analysis tools will read either format in all cases (that the author can think of!). Other analysis tools and visualization programs will vary, but in particular VMD will read both GROMACS trajectory formats.