There are several formulas available for the Penitential Act and apart from the Kyrie which remains the same there are changes for both priest and people.
In particular the words of the Confiteor (I confess) have changed:
The phrase ‘sinned through my own fault’ has been replaced with the two separate phrases. The first, ‘Sinned’ is now ‘greatly sinned’. This does not mean we are greater sinners than we were under the previous translation but reflects that our sins greatly offend God and have consequences, not only for ourselves but for others too. For example, the behaviour of someone who indulges in consuming too much alcohol has consequences for their own health and for others: a drink driver may injure or kill and innocent pedestrian; an individual could become violent in the street or in the family and those who drink heavily and often may put financial strain on their dependants, particularly children.
The second phrase includes the repetition of “through my fault” better reflects the Latin Mea culpa, mea culpa, mea maxima culpa and the three-fold denial of Peter during the passion of Christ.
The Collects, for example the opening prayer gather or ‘collect’ together all the prayers of the Mass said so far. These too have changed and are quite different to what we were used to under the previous translation.