A sigma bond has an orbital overlap directly between two nuclei while a pi bond has an orbital overlap on either side of the line between the two nuclei. Both sigma and pi bonds are covalent bonds since they both involve sharing electrons between two atoms.
Sigma bonds are stronger and more stable than pi bonds because there is more orbital overlap in sigma bonds. There can only be one sigma bond between two atoms while there can be zero, one or two pi bonds between two atoms. Single, double and triple bonds each have one sigma bond and zero, one or two pi bonds respectively. Sigma bonds determine the shape of the molecule. Pi bonds make atoms move closer together, and the combination of sigma and pi bonds creates a stronger and more stable connection between two atoms. Pi bonds form from the electrons in the parallel, leftover p-orbital of both atoms. Sigma bonds use hybrid orbitals. The electrons from pi bonds are much more likely to react with other atoms because they are less strongly bonded together. Because of their molecular geometry carbon atoms can have various hybrid orbitals and form single, double and triple bonds with other atoms. This versatility is one of the reasons carbon is the building block of organic compounds.