The principal parts of a verb are the four forms of the verb from which all forms of the verb can be made. In English the four principal parts are the present (or infinitive), the past tense, the past participle, and the present participle.
(To walk is considered a regular verb because we add a -d or -ed to the verb for the past and past participle.)
Principal Parts of the Verb To Walk:
(to) walk walked walked walking
(This is considered an irregular verb since one or more of the principal parts is formed in a nonstandard way.)
Principal Parts of the Verb To Run:
(to) run ran run running
Since the present participle is always formed by adding -ing to the infinitive, some lists of principal parts omit it.