Thermal Paste is used to bond an electronic component to a heat sink. The thermal paste is designed to conduct heat effectively. It is also an electrical insulator. The thermal paste has excellent adhesive properties. When a two surfaces such as a CPU and the heat sink are brought into contact, there are imperfections. These imperfections trap air bubbles and increase thermal resistance. 
When a thermal paste is used, it ensures that these imperfections are filled and heat transfer is proper.
Thermal pastes are usually made of a silicone base and zinc oxide, silver or ceramic.
Once applied, the thermal compound solidifies. The thermal paste can be removed later if required by using a cotton swab dipped in isopropyl alcohol.
Thermal paste is also known as Thermal grease.
The conductivity of the thermal matrix is indicated in watts/metre. kelvin