Just to clarify: The $50 option that most of us here call "confirmed standby" is actually referred to by UA as "same-day travel change". There's actually no standing by involved. You call UA up 4 hours before the flight you want, pay the $50, and you're confirmed on the flight.

The free option really is standby: You ask to be put on the flight after you show up at the airport. Your name goes on a standby list. There's no charge, but you might or might not make it on the flight.