Flood light bulbs that are made to be used indoors only are not made to be exposed to the elements such as rain, snow etc and should not be used outdoors because if they did they would be damaged and not work anymore once they get rain or snow on them.
The outdoor flood light bulbs are made for the outdoors but you can also use the outdoor flood light bulbs indoors as well as outdoors.
Outdoor Flood Light Bulbs are made to be exposed to the rain, snow etc and can get wet without hurting the bulb.