What are Bronsted acids and bases?

In short, acids are proton donors and bases are proton acceptors.

Take the following reaction for example:

#HCl + NH_3 rightleftharpoons NH_4^(+) + Cl^(-)#

Hydrochloric acid (#HCl#) is the Brønsted-Lowry acid because it donates a hydrogen ion. Ammonia (#NH_3#) is the Brønsted-Lowry base because it accepts the hydrogen ion.

The Brønsted-Lowry theory also introduces the concept of conjugate acid-base pairs. A conjugate acid-base pair are two species that differ by a (#H^(+)#) ion.

Based on the reaction above, the ammonium ion
(#NH_4^(+)#) is the conjugate acid of the base ammonia and the chloride ion (#Cl^(-)#) is the conjugate base of hydrochloric acid.

Here's a tip to let you know which substance is the conjugate acid and which substance is the conjugate base:

A conjugate base has one less (#H^(+)#) proton than the acid you started with.

A conjugate acid has one more (#H^(+)#) proton than the base you started with.

Here's a general depiction of conjugate acid-base pairs:

#HA# represents an acid

#B# represents a base

#A^(-)# represents the conjugate base since this substance has one less proton than the acid, HA.

#HB^(+)# represents the conjugate acid since this substance has one more proton than the base, B.

I really hope this makes sense!