A bustier is a combination between a bra and a corset. The difference being that a bustier has a wider bra band that can reach down to your ribs or waist. A bustier shapes your waist hence, the resemblance between a corset and a bustier. Most bustiers have a hooks and eyes closure that can be closed on the back.
Do you want to wear a bustier with cups? Then you can wear a bustier in your regular size. Is it a bustier without any cups? Then you can wear your regular shirt size!
You can wear a bustier the same way as your bra if you want to use it for the same purpose. Do you want to make a fashion statement? Wear your bustier as a top with a cardigan or wear it over your shirt!