What is Non-covalent interactions?

$\pi$ stacking is the attraction between flat aromatic rings lying face to face, usually offset or tilted. A cation–$\pi$ interaction is the attraction between a cation and the electron-rich face of an aromatic ring. A halogen bond is the attraction between the electron-poor tip of a covalently bound halogen atom, opposite its bond, and a Lewis base. The hydrophobic effect is the tendency of non-polar surfaces to come together in water, driven mostly by the release of the ordered water molecules around them.