SEVIS (Student and Exchange Visitor Information System) is the web-based system, run by the Student and Exchange Visitor Program (SEVP) within the Department of Homeland Security, that tracks F and M students, J exchange visitors and their dependents while they are in the US. Sponsors such as ECFMG enter your record and issue your DS-2019 from it. Each form carries a SEVIS ID, which starts with the letter N.
Most J-1 applicants must also pay the I-901 SEVIS fee, usually before the visa interview, and bring the receipt. Immigration authorities state the current fee and exemptions, so check the official site rather than a blog. Your SEVIS ID must match the one on your DS-2019 when you pay.
SEVIS is a record of your status, not a visa. If your program dates or details change, the sponsor updates SEVIS and issues a new form. See the J-1 visa process for where it fits in the sequence.