What is convection,conduction,radiation?

Conduction:
 it is the mode of heat transfer particularly in solids and also for liquid at rest. In this mode of heat transfer, the heat transfers from one atom to its neighbouring atom through molecular vibrations. At molecular level, First heat energy of a higher energy level  molecule converts to vibrating kinetic energy and this kinetic energy is transferred to neighbouring atoms and so on. again process repeats until the temperature difference between two neighbouring  atoms is zero. 

Convection:
This mode of heat transfer particularly occurs in fluids in motion. That is in both liquids and gases that are in motion. This mode of heat transfer occurs due to transfer of energy through bulk mass. 
In detail whenever there is temperature difference in a fluid, density difference occurs and motion of fluid starts as lower density fluid attempts to reach top of the fluid. During this motion mass and energy transfer occurs thus heat transfer takes place. 

Radiation:
In this mode heat can transfer even through vacuum. Heat transfer occurs as quantum packets or light energy. This is the mode by which we receive solar energy from sun.