A summary of the Fasting and Abstinence Regulations during Lent for Catholics:
(from this summary)
1) Abstinence on all the Fridays of Lent, and on Ash Wednesday and Good Friday
No meat may be eaten on days of abstinence.
Catholics 14 years and older are bound to abstain from meat. Invalids, pregnant and nursing mothers are exempt. (Fish and all cold blooded animals may be eaten, e.g., frogs, clams, turtles, alligators, etc.)
2) Fast on Ash Wednesday and Good Friday
Fasting means having only one full meal to maintain one's strength. Two smaller, meatless and penitential meals are permitted according to one's needs, but they should not together equal the one full meal. Eating solid foods between meals is not permitted.
Catholics who have reached the completion of the eighteenth year to the beginning of the sixtieth year are bound to fast. Again, invalids, pregnant and nursing mothers are exempt.
From the Code of Canon Law:
Can. 1249 The divine law binds all the Christian faithful to do penance each in his or her own way. In order for all to be united among themselves by some common observance of penance, however, penitential days are prescribed on which the Christian faithful devote themselves in a special way to prayer, perform works of piety and charity, and deny themselves by fulfilling their own obligations more faithfully and especially by observing fast and abstinence, according to the norm of the following canons.
Can. 1250 The penitential days and times in the universal Church are every Friday of the whole year and the season of Lent.
Can. 1251 Abstinence from meat, or from some other food as determined by the Episcopal Conference, is to be observed on all Fridays, unless a solemnity should fall on a Friday. Abstinence and fasting are to be observed on Ash Wednesday and Good Friday.
Can. 1252 The law of abstinence binds those who have completed their fourteenth year. The law of fasting binds those who have attained their majority, until the beginning of their sixtieth year. Pastors of souls and parents are to ensure that even those who by reason of their age are not bound by the law of fasting and abstinence, are taught the true meaning of penance.
Can. 1253 The conference of bishops can determine more precisely the observance of fast and abstinence as well as substitute other forms of penance, especially works of charity and exercises of piety, in whole or in part, for abstinence and fast.
From the US Bishop's Pastoral Statement on Penance and Fasting, pp. 10-15:
...We ask, urgently and prayerfully, that we, as people of God, make of the entire Lenten Season a period of special penitential observance. Following the instructions of the Holy See, we declare that the obligation both to fast and to abstain from meat, an obligation observed under a more strict formality by our fathers in the faith, still binds on Ash Wednesday and Good Friday. No Catholic Christian will lightly excuse himself from so hallowed an obligation on the Wednesday which solemnly opens the Lenten season and on that Friday called "Good" because on that day Christ suffered in the flesh and died for our sins.
In keeping with the letter and spirit of Pope Paul's Constitution Poenitemini, we preserved for our dioceses the tradition of abstinence from meat on each of the Fridays of Lent, confident that no Catholic Christian will lightly hold himself excused from this penitential practice.
For all other weekdays of Lent, we strongly recommend participation in daily Mass and a self-imposed observance of fasting. In the light of grave human needs which weigh on the Christian conscience in all seasons, we urge, particularly during Lent, generosity to local,national, and world programs of sharing of all things needed to translate our duty to penance into a means of implementing the right of the poor to their part in our abundance. We also recommend spiritual studies, beginning with the Scriptures as well as the traditional Lenten Devotions (sermons, Stations of the Cross, and the rosary), and all the self-denial summed up in the Christian concept of "mortification."
Let us witness to our love and imitation of Christ, by special solicitude for the sick, the poor, the underprivileged, the imprisoned,the bedridden, the discouraged, the stranger, the lonely, and persons of other color, nationalities, or backgrounds than our own. A catalogue of not merely suggested but required good works under these headings is provided by Our Blessed Lord Himself in His description of the Last Judgment (Mt 25:34-40). This salutary word of the Lord is necessary for all the year, but should be heeded with double care during Lent.