Karma yoga is the path of work without attachment: the karma-yogi must work without craving for the results of that work. Only when one works in this way can work be called yoga.
Sri Ramakrishna has some stern words of caution for the
karma-yogi:
It is not possible for you to give up work altogether. Your
very nature will lead you to it whether you like it or not. Therefore the
scriptures ask you to work in a detached spirit, that is to say, not to crave the
work’s results. For example, you may perform devotions and worship, and
practise austerities, but your aim is not to earn people’s recognition or to
increase your merit.
To work in such a spirit of detachment is known as
karmayoga. But it is very difficult. We are living in the Kaliyuga, when one
easily becomes attached to one’s actions. You may think you are working in a
detached spirit, but attachment creeps into the mind from nobody knows where.
You may worship in the temple or arrange a grand religious festival or feed
many poor and starving people. You may think you have done all this without
hankering after the results. But unknown to yourself the desire for name and
fame has somehow crept into your mind. Complete detachment from the results of action
is possible only for one who has seen God.
So the karma-yogi must be always on guard, always watchful,
for attachment creeps into the mind ‘from nobody knows where’. A good dose of
vairagya (detachment) is needed. And we can remember that, as Swami Vivekananda
says: ‘If working like slaves results in selfishness and attachment, working as
master of our own mind gives rise to the bliss of non-attachment.’
Source - excerpts from article titled 'Vairagya - Path To Freedom' by Swami Mahayogananda published in the Prabuddha Bharata Magazine January 2010 issue.