Spanish irregular verbs don't follow the patterns of regular verbs. That is to say, they change their stem (raíz) or their endings (desinencias).
Beber (to drink) is a regular verb of the 2nd conjugation. It follows the regular pattern on endings and never changes its stem:
Él, ella beb-e
Nosotros, nosotras beb-emos
Vosotros, vosotras beb-éis
Ellos, ellas beb-en
Caber (to fit) is an irregular verb of the 2nd conjugation. It follows the regular pattern on endings, but changes its stem:
Él, ella cab-e
Nosotros, nosotras cab-emos
Vosotros, vosotras cab-éis
Ellos, ellas cab-en
There are different types of irregular verbs: completely irregular, unique (only their derived verbs follow their pattern), common irregular verbs...