Asceticism is the practice of self-denial (i.e., control of one's passions and base impulses) for the sake of the Kingdom. The practice of asceticism - called ascesis - is most often associated exclusively with monasticism, although all the faithful are exhorted to practice lesser forms of ascesis through the Church's regimen of prayer, fasting, and repentance.
The word "ascetic" comes from the Greek root ἀσκητικός, which is turn is from the verb ἀσκέω, meaning "I train." The Apostle Paul likens the Christian life of prayer and repentance to training for various sporting events (1 Cor. 9:24-27; 2 Tim. 4:7). As such, the methods of ascesis should not be used as ends to themselves, but as means to the end of salvation, the "prize" which the Apostle mentions in First Corinthians.
Some forms of ascesis take a much more austere - even seemingly unhealthy - appearance than others, for instance Stylitism, in which the ascetic stands on a high pillar or tree for a prolonged period of time.
Suspicious of Pleasure?
Why is pleasure such a problematic issue in the Christian tradition?
The problem is about lust (the domination of the flesh) rather than pleasure per se. Pleasure itself is good, not a bad thing, but not something that should be an end in itself.
Not forgetting the natural connection between intercourse and procreation, it is only in marriage--a stable heterosexual bond marked by love and fidelity--that we can truly have intercourse that is for the good of the other person, as well as of any children who might be born.
Outside of marriage, sex is destructive. Desire for our own pleasure becomes a means by which we exploit others, defile ourselves by lust, and cause untold suffering to our children, should we have any (either by killing them, as in the case of abortion, or by raising them in a confused and chaotic situation). (I should also say that lust can destroy a marriage too.)
The effects of lust can be quite subtle, but in every case it is opposed to true 'love'. (Often when someone says 'I love you' they really only mean 'I lust after you'.) The motions of lust are self-centered and degrading to the full dignity of the human person.
This, then, is what is at the root of the Christian suspicion of 'pleasure.' Again, it is not 'feeling good' that is the issue, but the lust which disfigures.