A weak acid is an acid that dissociates less than 50% in water. The only acids that are not weak acids are strong acids. Weak acids include, but are not limited to:

Despite the name 'weak acids', the corrosive factor of an acid is not dependant upon how strong it is. Only how much an acid dissociates is dependant upon the strength. Hydrofluoric acid, for example, is a weak acid, but has the unique property of being able to etch glass.