A sigma bond is formed by end-to-end overlap of the bonding orbitals. Whereas, a pi - bond is formed by side-to-side overlap of (usually) two p-orbitals. This places the electron density above and below the plane of the nuclei with a node in the middle. The nodal region reduces the degree to which the orbitals overlap, and therefore, a pi bond is weaker than a sigma bond between the same two elements. Hydrogen bonds are a strong type of dipole-dipole interaction. As a Rule of Thumb, they are weaker than covalent and ionic ("intramolecular") bonds", but stronger than most dipole-dipole interactions.