Notes about Puranas:

Padma Purana, Uttara Khanda (236.18-21), explicitly states that the eighteen Puranas are divided according to the three modes of nature.:

vaisnavam naradiyanca tathabhagavatam subham garudanca tathapadmam varaham subhadarsane sattvikanipuranani vijneyani subhani vai brahmandam brahmavaivartam markandeyam tathaiva ca bhavisyam vamanam brahmam rajasani nibodhame matsyam kaurmam tathalaingam saivam skandam tathaiva ca agneyam ca sadetani tamasani nibodhame     * (Sanskrit - ancient Indic language, classical literary language of India )

Lord Siva said: "O beautiful lady (Parvati), know that the Visnu, Narada, Bhagavata, Garuda, Padma and Varaha Puranas are sattvika; the Brahmanda, Brahma-vaivarta, Markandeya, Bhavisya, Vamana and Brahma Puranas are rajasika; and the Matsya, Kurma, Linga, Siva, Skanda and Agni Puranas are tamasika."

Srila Prabhupada: Puräëas are made just to complete Vedic knowledge, or supplementary addition to the Vedic literature.Because Vedic literature is very difficult to understand, therefore they have been expanded by the Puräëas for different classes of men. So there are three divisions of the Puräëas: sattvika Puräëa, räjasika Puräëa and tamasika Puräëa. Sattvika Puräëa is meant for the higher class of people who are in transcendental knowledge of Brahman, Paramätmä and the Supreme Personality of Godhead. Räjasika Puräëa are meant for those who are aspiring to have improvement of material condition. And tamasika Puräëa are meant for those who are in the lowest stage of..., little, just like animal life, and to develop them the Puräëa is helping them to come to the second and first stage.

So there are different kinds of Puräëas, eighteen Puräëas. Because the whole Vedic literature means to claim all kinds of men. Not that those who are meat-eaters or drunkards, they are rejected. No. Everyone is accepted but there is-just like you go to a doctor. He'll prescribe you different medicine according to the different disease. Not that he has got one disease, one medicine. Whoever comes and, offers that medicine. No. That is real treatment. Gradually, gradually. But in the sättvika-puräëas, they are meant for immediately worshiping the Supreme Personality of Godhead. There is no gradual process. But gradually, one who comes to this stage, he's advised. So Padma Puräëa is one of the Puräëas in the modes of goodness.

"No one can understand the transcendental nature of the name, form, qualities and pastimes of Çré Kåñëa through his materially contaminated senses. Only when one becomes spiritually saturated by transcendental service to the Lord, are the transcendental name, form, quality and pastimes of the Lord revealed to him." (Padma Puräëa)

There are eighteen puräëas, six for each quality, modes of the nature. Six, six puräëas for the person who are in the modes of ignorance, six puräëas for the person who are in the modes of passion, and six puräëas for persons who are in the mode of goodness, those who are actually qualified brähmaëas. So Çrémad-Bhägavatam is called Mahä-puräëa. Mahä-puräëa means the topmost of all the puräëas.