It's insane how much I've learned about singing and being a performer in the past 8 months or so. I know this is hard to believe, but being a singer/musician is a lot like being an athlete:
I. Physical aspects
a) muscle memory--training your muscles to have an automatic response from repeatedly practicing the action. First you build the muscles, then they move automatically when required. In singers, these are mostly abdominal muscles and muscles around ur mouth/throat, which are not easy to control (when compared to the bicep, for example), and the movement/contraction should be almost involuntary.
b) stamina. My singing sucks if I haven't eaten, which is why I stockpile bananas. I know it's "just" singing, but it's so tiring... for example, a recital that is about an hour long is quite difficult for a singer.
II. Mental aspects
a) concentration--both athletes and singers have to be "in the zone" to perform well. There are all sorts of weird exercises to help them tune everything else out.
b) evil mental garbage--ever see the movie "Wimbledon"? Thinking you'll mess up, thinking you're not good enough, what ifs... affect performance. You know how just some days you can't filter through the garbage, whether you're working or eating or reading, etc?
Well right now I'm singing a very beautiful aria called the Bell Song, from Lakme. My last lesson did not go well, because I couldn't stop thinking about the People Who Owe Me Money.