I call these my "bra-dickies" because I wear them in place of camisoles under low cut sweaters and shirts for the layered look without the bulk.  They have a smooth seam line that doesn't give away that it's a bra, not a tank.

I would not recommend these for any sort of exercise if you have any amount of breast tissue because there is absolutely no support.  I'm a 34D and sometimes have to layer a regular bra underneath to prevent everyday bounce because the fabric is so stretchy.  Stairs are rough enough, I couldn't imagine attempting to run in it!