Most often, a Brønsted acid has a bond between H and some electronegative atom, such as a halogen, oxygen, nitrogen, sulfur, etc. Acid-base reactions can be thought of as a competition between two (or even more) electron-rich bases for the positive proton. The base that 'wins' this competition is the stronger base.

Roughly speaking, we can categorize acids as strong or weak. Strong acids are very much stronger than water or alcohol, and so the acid-base reaction with water goes essentially to completion (the acid is "completely ionized"). Weak acids are comparable in acid strength to water, and so the reaction with water is not complete (the acid is "partially ionized"). Of course, species without a bond between H and an electronegative atom almost never act as acids at all; that is, when dissolved in water (or alcohol), no proton transfer to the solvent occurs. The two species that are related to one another by the loss (or gain) of a proton are called a conjugate pair; thus, HA is the conjugate acid of A^- and A^- is the conjugate base of HA. When comparing two acids, we are really comparing two conjugate pairs. Consider two acids, HY and HZ. If HY is a stronger acid than HZ, then Y^- is a weaker base than Z^-.

Strong acid: HCl (hydrogen chloride or hydrochloric acid) is "completely ionized"

Weak acid: acetic acid is "partially ionized"

Not an acid: ethane does not donate a proton to water; no reaction occurs