If you think of this in binary terms, it means the first bit of a status byte is always 1. This makes it easy for the synth to determine what information it's getting, right from the first bit of a message. Similarly, all data bytes have a value of 127 or less, so their first bit would be 0. The first half of a status byte gives the command, and the second gives the channel. Whenever you send a noteon, you should send a noteoff when you want it to stop.
Channel pressure is similar to key pressure, but applies to all notes on a given channel.
Pitch bend is usually generated by a wheel or joystick, with no bend in the middle of the wheel. So 0 represents maximum bend down, and 16,393 represents maximum bend up. Controllers might also select a different bank of sounds, to change the sound that's playing. It's useful for when you've got notes that you can't stop playing and can't figure out why. No second data byte is necessary.