Orchestra is a large group of musicians that play together using different musical instrument like strings, brass instruments, percussion instruments and wood wind instruments. It is a collection of musicians and musical instruments led by a conductor. Orchestra play all type of music including classical.
Symphony is the common classical that is played by orchestra while Opera is a theatrical work that combines drama, song and sometimes dance. In opera, orchestra plays the music and the vocalist sings and act on the stage. There is no speaking in opera, the vocalist sings all the text.