Grand opera has a very serious theme. Every word in the play is sung by the actors, no one speaks. Here are some famous grand operas:

Amahl and the Night Visitors
Madame Butterfly
The Magic Flute
Romeo and Juliet
Marriage of Figaro

Light opera has a humorous or romantic theme. Some words are spoken and some are sung. Sometimes it is called comic opera or an operetta. You may have heard of some of Gilbert and Sullivan's light operas:

The Pirates of Penzance
Mikado
The Merry Widow
HMS Pinafore