A nitrile contains a triply bonded C=N group, which is another common type of polar bond.  However, the carbon in a nitrile is at an oxidation state of +3, higher than the +2 for ketones or +1 for aldehydes.  For this reason, nitriles are not carbonyl equivalents like imines; rather, they are at the same oxidation state as carboxylic acids.

The C=N triple bond undergoes nucleophilic additions in a similar way to a C=O.  As with the carbonyl group, so often protonation is needed in order to activate it for the addition of weak nucleophiles such as water.  As expected, when nitriles are hydrolyzed (by nucleophilic addition of water followed by nucleophile elimination), they form either carboxylic acids or amides, depending on the conditions.

General reaction

Water is used along with the acid, to provide the reagent for the hydrolysis.