Roth IRAs contain post-tax dollars (you make deposits using money you have already paid taxes on).
Roth IRAs grow tax-free.
Roth IRA contributions can be withdrawn at any time, penalty-free and tax-free. There is no age requirement. However, Roth IRA earnings and rollovers cannot be withdrawn before age 59.5, or they are subject to a 10% penalty (still no taxes though).
Vanguard is never a bad choice.
Robo-advisors are good if you are just getting started, or if you want a hands-off approach.
Index funds are the name of the game.
Target date funds are a great way to have a mix of index funds and bonds that gradually shifts towards the safety of bonds as you get older.