With a Straight/Box bet, half of the stake goes on a Straight bet and the other half on a Box. You win just the Box bet if you match the winning numbers in any order, or you can win both parts of the bet by matching the numbers in the same order.
When you play a Combo, you pick three numbers and then pay for a Straight bet for each possible combination of your numbers. This is the most expensive type of bet but also slashes the odds of winning from 1 in 1,000 (the odds of winning a standard Straight bet) to 1 in 333 for a 3-way Combo and 1 in 167 for a 6-way Combo.