There are two common touch-up "tools" that will do the job on most minor knicks on a cabinet finish - fill sticks and stain pens.  In general stained wood finishes are the easiest to touch-up.  

A fill stick is almost like a wax crayon.  It is simple to use just rub the fill stick on the area and rub off any excess with a soft cloth.  As they are only effective on a small mark, there is no need to put any type of finish coat on top of the fill stick.

Stain pens can be used to fix-up a little larger area where the finish has been rubbed through to the bare wood.  Stain pens are similar to a felt pen.  Simply run the felt tip over the defect area.  Remove any excess with a soft cloth.  It may take a couple of applications to get the deisred coverage.  Do not let the stain dry before wiping off any excess.  If the defect area being fixed-up is fairly large and/or very prominent, it may require a coat of clear lacquer on top of the stain so the finish more closely matches the original factory finish.

Attempting to fix up a solid lacquer finish that has been damaged is the more challenging touch-up.  For very small minor knicks the fill stick may do the job but large marks are unlikely to be repaired.  While the mark may be covered up, it is likely that the mark will be just as noticable.