Exotics are oragnisms that have been introduced by human activity into an ecosystem in which they are not native. Naturalized species are introduced species that have been present so long among their associates that mutual co-existence is demonstrated. It's quite clear from these definitions that exotic species have been introduced by humans. There are four ways that this happens. I'll give examples of each.
Invasive species can cause several different kinds of changes in ecosystems.
They can out-compete native species and cause their local extirpation, thus reducing biodiversity. They can cause diseases in native organisms. For example, Dutch elm disease and chestnut blight are both problems brought on by non-native species. They can prey on native species and cause extirpation or population declines. They can degrade habitats and reduce biodiversity through erosion or other means.