Acid is a proton donor and based is proton acceptor.
Any compound that has a hydrogen atom can act as a Bronsted-Lowry acid.
Hence H-A is used as general representation for an acid (H represents hydrogen)
According to Lewis theory, any molecule that has an electron deficient atom (site) that can accept a pair of electrons, is an acid.
The effects of structure of the molecule on the strengths of acids and bases
Acids and Bases - Videos on Knol