Hydrofluoric Acid is classified as a weak acid because of its lower dissociation constant compared to the strong acids. It ionizes in aqueous solution in a similar fashion to other common acids:

HF + H2O  H3O+ + F−

It is the only hydrohalic acid that is not considered a strong acid, i.e. it does not fully ionize in dilute aqueous solutions.

When the concentration of HF approaches 100%, the acidity increases dramatically because of homoassociation:

3 HF  H2F+ + FHF−

The bifluoride (FHF−) anion is stabilized by the very strong hydrogen–fluorine hydrogen bond.

Source: Wikipedia.com