There is quite a range of ingredients in a degreasing product, and each one will have a slightly different mixture. Even where they share ingredients, they will often be in slightly different quantities across the various brands.
Broadly speaking however these products contain some kind of agitator that works to break down the grease, oil and dirt as well as a solution (normally either petroleum or water based) into which the agitating ingredients are suspended.
The cleaning ingredient(s) differ from product to product and are often a closely guarded secret held by each manufacturer. In our list above you can see some products that include an Emulsifying agent to quickly break down grease and oil and make them easy to wash away. We even have a product that uses Citrus as its cleaning agent. Each manufacturer may not specify exactly what the cleaning agent is, but broadly speaking they all perform the same function.