Caulk is an old boat-building term; sealant originated in home building. Today, some manufacturers use caulk as an all-purpose term and sealant to describe their high-performance products. Most often, though, the terms are used interchangeably, and the products serve the same purpose: to fill gaps between building materials and to keep water and air at bay.
Just as they do the same job, caulks and sealants fail for the same reasons. There are three types of caulk failure: adhesive, cohesive, and substrate. Simply put, the bond between the caulk and the substrate can fail, the caulk itself can tear, or the substrate can break. However, problems with caulked joints are commonly due to one of two errors. Either the substrate was not effectively prepared, or the wrong product was selected. So before choosing a product, consider how it will be used.
First, consider what materials the joint is made of and how much movement it is likely to encounter. Silicone, for example, adheres well to glass and tile but poorly to wood. Although products with different chemistries claim to be flexible, some are better suited for frequent joint movement. Remember, too, that caulk generally is not recommended for gaps that exceed 1/2 in. wide at their midpoint.