At their most simple, adhesives are substances used for sticking objects or materials together. In scientific terms, those substances that are sticking together, regardless of what they are, become known as "substrates."
Think of a peanut butter and jelly sandwich. You could say the peanut butter and jelly is an adhesive for the two pieces of bread, the substrates.
Many adhesives are made for specific purposes, depending on the industry and the substrates that need to be bonded together. For instance, there might be a special glass adhesive for a car window, but a completely different one for a coffee table. Each has its own unique formula and is specifically designed for a particular need.