Question

What’s the difference between adding a JAR to the the CLASSPATH of a project in the IDE and adding it to the system wide CLASSPATH?

Answer 1

The latter is valid for ALL java applications you are running while the former only for the project.
I’d recommend not using the system wide classpath as things tend to get rather confusing and messy very quickly.

From here:

PATH and CLASSPATH

Checking the CLASSPATH variable (All platforms)

The CLASSPATH variable is one way to tell applications, including the
JDK tools, where to look for user classes. (Classes that are part of
the JRE, JDK platform, and extensions should be defined through other
means, such as the bootstrap class path or the extensions directory.)

The preferred way to specify the class path is by using the -cp
command line switch. This allows the CLASSPATH to be set individually
for each application without affecting other applications. Setting the
CLASSPATH can be tricky and should be performed with care.