Sample dress are the ones already in the store for brides to try on to get a sense of how the dress will look on your body. Most boutiques will only have one size of a specific gown and typically a size 8, 10, 12 (bridal sizes on average are 1 to 2 sizes smaller than street sizes) and sometimes larger sizes are available to accomodate full figured brides, but unfortuntaley plus size are a more limited inveontry.
Samples are usually sold to make room for new styles and can be discounted anywhere from 20% to 70% dependent on the age of the gown, popularity (or lack there of) and condition (rips, tears, stains missing buttons, broken zipper etc.). Samples under 12-18 months old are typically not sold, since having them in the bridal boutique is the best form of marketing and generating salels, this is especailly true for poular gown. Unless you are specifially buying a sample, you will receive a new dress for your wedding day + the cost of alterations.