What two important differences are there in the properties of sigma and pi bonds, in terms of how they affect
the structure and reactivity of molecules?

A triple bond is one sigma and two pi bonds. A sigma bond is your basic head-on covalent bond, with the bond in line with the bonding orbitals. You can only ever have one sigma bond between any two atoms. A pi bond is a covalent bond between orbitals perpendicular to the bond direction, usually p-orbitals (never s).