What is the difference between carboxylate ions and groups?

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Carboxyl

The carboxyl group $\ce{-COOH}$ is an organic functional group consisting of a carbon atom double bonded to an oxygen atom and single bonded to a hydroxyl group.

Compounds with a carboxyl group are called carboxylic acids (e.g. acetic acid, $\ce{CH2COOH}$).

Carboxylate

A carboxylate $\ce{-COO^-}$ is a salt or ester of a carboxylic acid. For example, acetic acid reacts with sodium bicarbonate to form sodium acetate.

Compounds with this group have the carboxy- prefix or -ate suffix (e.g. sodium acetate $\ce{CH3COONa}$). When these salts are ionised (e.g. dissolved in water), the anion with the carboxylate group is called a carboxylate ion.

The carboxylate ion is the negative ion (anion) formed as the conjugate base when a carboxylic acid loses a proton.

$$\ce{R-COOH -> R-COO^- + H+}$$

Being its conjugate base, sodium acetate mixed with acetic acid makes a useful buffer solution for many biochemical reactions where a ‘mildly acidic’ pH of 4-6 is desired.