Are acids proton borrowers or donors?

Acids are proton donors. Bases, on the other hand, are proton borrowers or acceptors.
When an acid is dissolved in water, it donates the protons, leading to the formation of hydronium ion and an anion. For example, the reaction between the hydrochloric acid and water can be written as:
HCl + H_2O -> H_3O^+ + Cl^-
Here, hydrochloric acid is the acid since it donates a proton, while water is the base as it accepts a proton. 
Based on their ability to donate the protons, the acids are classified as either monoprotic, diprotic, or triprotic acids. Monoprotic acids (such as hydrochloric acid or nitric acid) may donate one proton per acid molecule. Diprotic acids (such as sulfuric acid) may donate up to two protons per acid molecule. Similarly, triprotic acid (such as phosphoric acid) may donate up to three protons per acid molecule.
Hope this helps.