An Arrhenius base is a substance that contains an OH group and produces hydroxide ions OH - in aqeuous solution. The Arrhenius theory is mainly of historic interest, and we will not use it further. A Bronsted-Lowry Acid is the entity which donates the proton. A Bronsted-Lowry base is the entity which accepts the proton. This base and its conjugate acid is, again, a conjugate acid-base pair .
Acids differ in how readily they donate the proton.
Likewise, bases differ in how readily they accept the proton. The acid strength of an acid is related to the basic strength of it conjugate base. The stronger an acid is, the weaker its conjugate base. Likewise, the stronger a base, the weaker its conjugate acid. Sometimes the same material will acid both like an acid and a base in the same reaction.