An invasive species is a species which has moved from its native area, where it grew naturally, into a new area. Often it becomes a nuisance species, because in its new habitat it lacks its old enemies. Normally, as a species evolves, so do the things which eat it or parasitise it. Free of natural restraints, the invading species outcompetes its new neighbours.
When a species is wiped out is does not necessarily bring things back to normals. The animals might have adapted to the invading species or the invading species have some how become part of the ecosystem's food web.