So...I left something out. Something BIG! Wagner's Leitmotifs - or leading/guiding motives. Basically Wagner wrote little themes or motives into the orchestra to signify a character, a place or an idea. (He wasn't the first to do this, but certainly the most prominent.)
These motifs are usually played in the orchestra the first time we see a character, place or idea and then in a slightly altered form later in the opera or later in the cycle of operas. Wagner uses the motifs to clue the audience in on what is really going on. For example in Das Rhinegold, we are introduced to Wotan's motif. In the second opera, Die Walkure, Seigmund is talking about his father whom he calls Wolfe. But then the orchestra plays Wotan's motif, and we the audience know who his father really is, while the character is still clueless.
These various motifs tie all the operas together in such an amazing way. And often, when we hear the motif again, it's in a slightly altered form - not always overtly noticeable. This is part of the wonder of Wagner's music. Movies and musicals have taken their cue from this i.e. Star Wars, Lord of the Rings; Phantom of the Opera, Doctor Who - but Wagner had a finesse about him, that has not been paralleled.
By the way, I've read that Wagner didn't like the term Leitmotif but instead preferred Hauptmotiv...didn't stop everyone from using the former.
