Bootstrap Wells class create an inset effect on the page apply .well class on the container <div> then inset effect will appear on that. Simple method to create a well, wrap your content where you would like to appear in the well with a <div> with the class of .well. The following example shows how it looks like.
There are two classes which use for change the size of well .well-lg and .well-sm. These both classes are used in combination with .well class. These have an effect on the padding and making the well larger or smaller depending on the class.