The difference between java application library and java web lib

I remebered that i had writed a java web program.At that time, i added jar packages by "build path" so that i could import a user library path at firstly.

When the path of jar library had been imported into eclipse,to complise programme was right, but it was wrong to run at the Tomcat server.
The Server would throw a wrong information that could not find class api in the jars.

So i would like to know what leads the condition. Then i found the answer on the web,finally, i summary following contents by different answers some people on the web.

I have known that eclipse project only copys the path of jar library,so it shall not add these jars into the project.Then i can program rightly in the editor.

But i must run web project on the server that has own class loader. And the server to load jars at the lib folder that is under WEB-INF.Therefore, we have to add outer jar packages into the lib folder when we editor Web program.

Finally, it leads to a condition that we can compile program in Eclipse,but can not run on the server cause of finding not jar packages when we import
external jar packages by Eclipse's build path function at programming web program.

Notice: i have to obye the rulers that special framework, beacuse different frameworks have different running mechanism.