Question

What is the different between different kind of Java EE projects in Eclipse WTP?

There are:

• Application Client Project
• Connector Project
• Enterprise Application Project
• Utility Project

Answer 1

Quoting the Chapter 5. Projects of the RAD v7, Programming Guide (applies to Eclipse too):

• Application Client Project (J2EE) — Guides the user through
  creating an empty Application Client
  project. Includes prompts for the
  associated EAR project and a list of
  facets applicable for J2EE Application
  Client projects.
• Connector Project (J2EE) — Guides the user through creating a
  J2EE connector project, which includes
  specifying the associated enterprise
  application project and a set of
  facets applicable to it.
• Enterprise Application Project (J2EE) — Creates a new EAR project.
  This includes options for creating
  associated Web, EJB, and Application
  Client projects.
• Utility Project (J2EE) — Assists in the construction of a Java utility
  library project which is associated
  with an Enterprise Application
  project. The product of such a project
  is a JAR file that is available to the
  resources within the Enterprise
  project and is automatically wrapped
  into the EAR file when it is exported.

This is detailed a few pages later:

Enterprise Application project

Enterprise Application projects contain the resources needed for enterprise
applications and can contain references to a combination of Web modules, EJB
modules, application client modules, resource adapter modules, utility Java
projects, and JAR files.

These relationships can be specified when creating a new Enterprise Application
project through the wizard or through the project properties.

For more information on developing J2EE enterprise applications, see
Chapter 16, “Develop Web applications using EJBs” on page 719.

J2EE Application Client project

J2EE Application Client projects contain the resources needed for J2EE
application client modules. An application client module is used to contain a
fully-functioning client Java application (non-Web-based) that connects to and
uses the J2EE resources in a J2EE enterprise application and an application
server. By holding a reference to the associated enterprise application, it shares
information such as the Java Naming and Directory Interface (JNDI) reference to
EJBs and data sources.

Connector project

A connector project contains the resources required for a J2EE resource adapter.
The wizard allows a set of facets (including the J2EE Connector Architecture
(JCA) version) and containing EAR file to be specified.

Short version: they are all wizards to create an Enterprise Application or its parts.