What are linking verbs?

A linking verb joins a subject to a subject complement (also called a predicate noun or predicate adjective), a word or phrase that describes or explains the subject. The most common linking verb is the verb to be.
Kayaking is my favorite sport.
My grandmother was an expert pilot.

Other common linking verbs are seem, appear, look, feel, sound, taste, and smell.

This shirt feels comfortable.
That cake smells delicious.

Linking verbs are always intransitive. If a verb takes a direct object, it is not a linking verb.

The dog smelled the garbage. (Transitive verb with direct object garbage.)
The garbage smelled awful. (Linking verb to subject complement awful.)