A novena is 9 days or 9 hours of a set of prayers for a particular devotion (A Saint or Mary, Or the Sacred Heart, etc.) for a specific intention.
A rosary can be prayed as a Novena…but in general, it’s a rosary that stands alone. Many people pray a rosary a day and add in their intentions.
A novena is a series of prayers that are said for nine straight days, usually as a prayer of petition but sometimes as a prayer of thanksgiving. (See The Types of Prayer for more on prayers of petition and thanksgiving.) The nine days recall the nine days that the Apostles and the Blessed Virgin Mary spent in prayer between Ascension Thursday and Pentecost Sunday.
While the word novena comes from the Latin novem, meaning “nine,” the term has also come to be used generically to refer to any lengthy series of prayers.
For example, the Saint Andrew Christmas Novena is recited for more than nine days, between the Feast of St. Andrew (November 30) and Christmas.