In the present tense it is regular, but we generally pronounce the present tense third person singular form, which takes a third person -s, "says" as /sez/ rather than /seɪz/. In the past tense it is an irregular lexical verb. "Said" (/sed/) is both the preterite and the past participle, the present participle is saying. It is a weak verb and follows a similar pattern to lay (lay-laid-laid) and pay (pay-paid-paid), but not bay, way or flay, slay, stay, etc, which are regular, or indeed may which is a full modal verb.
It can be used as a noun, or adverb.
See also saying