What is Opera Seria?

Opera seria is a genre of opera. It was common in the 18th century. The term itself is Italian, but opera seria was performed in many European countries in addition to Italy. The style of this kind of opera has a more serious tone, as opposed to more comical and lighthearted operas.  
This type of opera was typically enjoyed by the more educated population, such as nobility. Opera seria was popular during the time of High Baroque music. Handel and Mozart were two famous composers who created music for operas in the opera seria genre.  
The genre usually centered around tragedy. Order and structure were also important. Operas in the opera seria genre had small casts of characters, rather than large choruses. Complicated arias were a key part of opera seria. These complexities showed off various vocal types, from a high-pitched male voice to a prima donna's voice.