A regular verb is any verb whose conjugation follows the typical pattern, or one of the typical patterns (“-s”, “-ed”). A verb whose conjugation follows a different pattern is called an irregular verb.
Verbs such as play, associate are regular, since they form their inflected parts by adding the typical endings -s, and -ed, to give forms such as plays, associated.
On the other hand, verbs such as drink, hit and have are irregular, since some of their parts are not made according to the typical pattern – drank and drunk (not “drinked”); hit (as past tense and past participle, not “hitted”) and has and had (not “haves” and “haved”).
Please note that many of the most frequent verbs are irregular. Thus it is essential to learn all the English irregular verbs.