Tafseer is the explanation of the Quran. Its basic function is to provide explanations of the events in which verses were revealed, the times they were revealed in and the places. A tafseer also provides different views of the scholars on the verses of the Quran. Tafseer today is considered as a science and there are many Quran classes online where a person can learn Quran online and learn Quran with tafseer. People who are in non-Muslims areas of the world tend to learn Quran online and learn Quran with tafseer through these Quran classes online, however it must be known that tafseer is a very wide subject and it is imperative to learn it in detail and attending an institute that provides in depth courses regarding it is recommended.
In the Early Days of Islam
In the days of the Prophet Muhammad (SAW), the science of tafseer had already begun. Prophet Muhammad (SAW) is in fact of the main and primary fountains of knowledge that explained the verses of the Quran to the people. During this time, tafseer of the Quran was a relatively easy matter simply due to the fact that Prophet Muhammad (SAW) was present among the people. Another aspect that made it easy was the fact that the companions were witnessing the revelation of the Quran and were mostly aware of why verses were being revealed. Another major reason was the Arabic used in the Quran was the Arabic that was spoken commonly so it was easy to understand.
After the Prophet Muhammad (SAW)
After the time of Prophet Muhammad (SAW), it was the companions that took a more systematic approach to the tafseer of the Quran. The sources that the companions used for this were the Quran, the sayings of the prophets, the principles of the Arabic language and their own personal reasoning (ijtihaad) along with the pagan, and Judaeo-Christian that were prevalent in the times when the Quran was being revealed. Apart from the companions there are others who also are well known for their tafseers such as Imam Ibn Kathir whose tafseer is considered to be one of the best and most reliable.