Classpath in java and spring

I am majorly confused about where the classpath is. I understand when we create a spring mvc, resources folder, or inside web-inf is considered classpath. And we can use "classpath:" inside xml files to declare the folder. However, where is this classpath exactly? How is it set ? I have been reading about it for a long time, i still couldnt manage to get a real clear image in my head how the classpath is initially determined etc.

For example when we create a war file, and deploy it on a tomcat server, all the resource files can still be read via given paths with "classpath:" in the xml files. How does this work?

Thanks.