The Sixth House is related to things which we do through a sense of duty, such as our work. It is not really concerned with our career (which is more the province of the Tenth House) but rather the day-to-day experiences which we have in our working lives. It is also related to our health, and to any relationship which is unequal, and where there a sense of duty is involved. In addition, it is related to hygiene, and to small animals.
The planets in a person's Sixth House show the way in which he or she is able to use these corresponding energies in work. They might also indicate the type of work done; but more likely, they will show the type of energy which we bring to work. For example, Mercury in the Sixth House would suggest a link between communications and work, showing that this person did work which used his or her ability to communicate ideas with other people.