(1) Abstinence of sex. V-Raamaayan (1/9) says There are two types of Brahmcharya - Vrateetwa and Prajaapatya. In Vrateetwa, the person does not marry at all; and in Prajaapatya, the person keeps away from his wife on certain days.

(2) Bahmcharya is a compound word, Brahm + Charya. Brahm means Bhagavaan and Charya is conduct, behavior, attitude, disposition, demeanor, and Brahm is the Truth. The conduct of reality is Brahmcharya; thus Brahmcharya means "the conduct of Brahm".

So, when you conduct yourself in a manner not in contradiction to the nature of Truth, you are supposed to be observing Brahmcharya. And what is the nature of Truth which you should not contradict in your day-to-day conduct and which is supposed to be Brahmacharya? The nature of Truth is non-sensory existence. Truth is not a sensible object. It is not seen, it is not heard, it is not tasted, it is not touched, it is not contacted by any of the senses of our individual personality. Therefore, to desire for the objects of sense would be a contradiction of the nature of Truth. Brahmcharya is sensory non-indulgence. The opposite of sensory indulgence is the attitude of Brahmcharya.

Our present-day activities are mostly a refutation of the principles of Brahmcharya, and so we are weak in every respect. We are unable to see, unable to hear, unable to touch, unable to walk, unable to speak, unable to digest our daily meal. Everything has been weakened, because our senses refute the existence of God. When you see an object you deny God, because the denial of God and the perception of an object are one and the same thing. When you hear a sound, you deny God. When you taste, when you touch, when you have any kind of sensory activity there is an unconscious refutation of the indivisibility of the existence of God.

Brahmcharya has thus been, by an extension of its meaning, regarded as sense-control. But sense-control is not the whole meaning of Brahmcharya. It is a spiritual attitude to things that is called Brahmcharya, which implies, of course, automatically, sense-control. When it is day-light, when the Sun is up above our heads, it is understood that darkness has gone. But day is not merely the absence of darkness. It is a positive kind of enlivening and energizing phenomenon, a power that we receive from the Sun, including light.

So, Brahmcharya is not merely a withdrawal of the senses from contacts with objects, though it implies that also. It is an inward positivity of attitude. In Brahmcharya, you become a positive person, with a content of your own, independent of any kind of external aid. You have a stuff of your own, as they call it. That is called Brahmcharya.
[Taken from "The Secret of the Katha Upanishad"]