gmx traj plots coordinates, velocities, forces and/or the box. With -com the coordinates, velocities and forces are calculated for the center of mass of each group. When -mol is set, the numbers in the index file are interpreted as molecule numbers and the same procedure as with -com is used for each molecule.
Option -ot plots the temperature of each group, provided velocities are present in the trajectory file. No corrections are made for constrained degrees of freedom! This implies -com.
Options -ekt and -ekr plot the translational and rotational kinetic energy of each group, provided velocities are present in the trajectory file. This implies -com.
Options -cv and -cf write the average velocities and average forces as temperature factors to a .pdb file with the average coordinates or the coordinates at -ctime. The temperature factors are scaled such that the maximum is 10. The scaling can be changed with the option -scale. To get the velocities or forces of one frame set both -b and -e to the time of desired frame. When averaging over frames you might need to use the -nojump option to obtain the correct average coordinates. If you select either of these option the average force and velocity for each atom are written to an .xvg file as well (specified with -av or -af).
Option -vd computes a velocity distribution, i.e. the norm of the vector is plotted. In addition in the same graph the kinetic energy distribution is given.