I’ve heard both terms used and wonder what is the distinction between the two.
A missal is a prayerbook that contains all of the readings and prayers for all of the Masses throughout the liturgical year. Missals usually are printed in separate Sunday and daily versions. The Sunday missals contain all of the Sunday readings and the daily missals contain all of the daily readings. If a Catholic has a personal missal, he can use it year after year without having to rely on the parish to supply the readings for the Mass. The only time a missal would become obsolete is when the Mass significantly changes, as it did after the changes to the ordinary form of the Roman rite after Vatican II.
A missalette is a prayerbook provided by a parish that usually contains the prayers for the Mass, the Sunday readings for the liturgical season, and some seasonal hymns that may be used by the cantor or choir. The readings are usually arranged by the specific date on which they will be read, and because of this the missalettes are obsolete after the liturgical season is done. The parish then buys a new batch of missalettes for the next season. It is called a missalette because it is a smaller version of a standard missal.