The ingredients of a degreaser can vary wildly depending on the product. Generally speaking, they fall into 2 camps:
1) solvent cleaners: this includes alcohols (like isopropyl alcohol, or ethyl alcohol), hydrocarbons (like heptane and mineral spirits), ketones (like acetone and xylene), and more exotic compounds and blends.
2) water-based cleaners: these include ingredients dissolved or blended with water. Which is best for your application depends on the type of soil and various requirements like performance, evaporation rate, toxicity limits, and environmental regulations.