The Eclipse plugins generate files that are used by the Eclipse IDE, thus making it possible to import the project into Eclipse (File - Import…​ - Existing Projects into Workspace).

The eclipse-wtp is automatically applied whenever the eclipse plugin is applied to a War or Ear project. For utility projects (i.e. Java projects used by other web projects), you need to apply the eclipse-wtp plugin explicitly.

What exactly the eclipse plugin generates depends on which other plugins are used:

Table 1. Eclipse plugin behavior
Plugin Description