ISIS is a terrorist group that sprung off of Al-Qaeda in Iraq after the invasion from the United States left the region destabilized and with a power vacuum. However, ISIS is not exactly Al-Qaeda itself, as ISIS brutality has shocked even some Al-Qaeda branches and affiliates.
ISIS stands for Islamic State of Iraq and Syria. This is not the only name the organization is called though. Sometimes, President Obama has referred to the group as ISIL, which is more Islamic State of Iraq and the Levant. The Levant is more the greater Middle East, and it is speculated that this reference correlated with early efforts to focus against ISIS only in Iraq, and not also in Syria, although this has changed since then.
The group nowadays simply calls itself IS or Islamic State. Many Arab nations call the group "daesh," which is something of an insult to the group, even mocking in tone and history to the group's earlier names. It also sounds similar to an Arab word meaning to crush underneath a foot, or to stomp.
As an uncapitalized word instead of an acronym, Isis can also refer to goddess within the ancient Eygptian pantheon of deities, whose name roughly meant "throne." This is seemingly unrelated to the modern terrorist group, although she has some followers and worshippers in countries whose history were impacted by Greco-Roman traditions.