Devotional service means to prosecute Kṛṣṇa conscious activities which are favorable to the transcendental pleasure of the Supreme Lord Kṛṣṇa, and any activities which are not favorable to the transcendental favor of the Lord cannot be accepted as devotional service. For example, great demons like Rāvaṇa, Kaṁsa and Hiraṇyakaśipu were always thinking of Kṛṣṇa, but they were thinking of Him as their enemy. This sort of thinking cannot be accepted as bhakti or Kṛṣṇa consciousness.
The impersonalists sometimes misunderstand devotional service in such a way that they divide Kṛṣṇa from His paraphernalia and pastimes. For example, the Bhagavad-gītā is spoken on the battlefield of Kurukṣetra, and the impersonalists say that although Kṛṣṇa is of interest, the battlefield of Kurukṣetra isn't. The devotees, however, also know that the battlefield of Kurukṣetra by itself has nothing to do with their business, but in addition they know that "Kṛṣṇa" does not mean just Kṛṣṇa alone. He is always with His associates and paraphernalia. For instance, if someone says, "Give something to eat to the man with the weapons," the eating process is done by the man and not by the weapons. Similarly, in Kṛṣṇa consciousness, a devotee may be interested in the paraphernalia and locations—such as the battlefield of Kurukṣetra—which are associated with Kṛṣṇa, but he is not concerned with simply any battlefield. He is concerned with Kṛṣṇa—His speech, His instructions, etc. It is because Kṛṣṇa is there that the battlefield is so important.
This is the summary understanding of what Kṛṣṇa consciousness is. Without this understanding one is sure to misunderstand why the devotees are interested in the battlefield of Kurukṣetra. One who is interested in Kṛṣṇa becomes interested in His different pastimes and activities.
The definition of a pure devotee, as given by Rūpa Gosvāmī in Bhakti-rasāmṛta-sindhu, can be summarized thus: his service is favorable and is always in relation to Kṛṣṇa. In order to keep the purity of such Kṛṣṇa conscious activities, one must be freed from all material desires and philosophical speculation. Any desire except for the service of the Lord is called material desire. And philosophical speculation refers to the sort of speculation which ultimately arrives at a conclusion of voidism or impersonalism. This conclusion is useless for a Kṛṣṇa conscious person. Only rarely by philosophical speculation can one reach the conclusion of worshiping Vāsudeva, Kṛṣṇa. This is confirmed in the Bhagavad-gītā itself. The ultimate end of philosophical speculation, then, must be Kṛṣṇa, with the understanding that Kṛṣṇa is everything, the cause of all causes, and that one should therefore surrender unto Him. If this ultimate goal is reached, then philosophical advancement is favorable, but if the conclusion of philosophical speculation is voidism or impersonalism, that is not bhakti.
Karma or fruitive activities are sometimes understood to be ritualistic activities. There are many persons who are very much attracted by the ritualistic activities described in the Vedas. But if one becomes attracted simply to ritualistic activities without understanding Kṛṣṇa, his activities are unfavorable to Kṛṣṇa consciousness. Actually, Kṛṣṇa consciousness can be based simply on hearing, chanting, remembering, etc. Described in the Śrīmad-Bhāgavatam are nine different processes, besides which everything done is unfavorable to Kṛṣṇa consciousness. Thus, one should always be guarding against falldowns.