A placket, when used to describe a part of a dress shirt, is the strip of fabric that runs along the front of the shirt and is used as a receptacle for the buttons responsible for closing the shirt.
There are three main types of plackets that are commonly found on dress shirts; standard, concealed and seamless. A seamless placket, also known as a French placket, is the most popular of the three due to its simple and classic appearance. The seamless placket is nearly indistinguishable from the rest of the shirt and presents a very clean front that minimizes any stitching.
The notion of a clean front is taken one step further with a concealed placket. A concealed placket uses an additional strip of fabric to cover the buttons thus hiding them from view. Wing-tip collar tuxedo shirts worn with bow ties at black tie invitations are one of the more common examples that frequently incorporate concealed plackets.
The last type of placket, the standard placket, emphasizes the stitching used to attach the placket as well as this strip of fabric used for the placket itself. Rather than attempting to hide the inner workings of the shirt; the standard placket emphasizes the placket and the stitching as a design element.