The word tafsir means commentary; it is refereed usually to the commentary of Quran. The source for commentary of the Quran are the Quran (one verse of Quran is usued to explain another), the hadits, the reports of the Sahabah and the reports of the Tabiun.
My favorite book of tafsir is the Maariful Quran. Here I added the link of another website from where the visitors can download Maariful Quran. I also upload Tafsir Ibn kathir.
