in php what is the differnce between :: and ->?

Hey...so Im not fully gettin it..if i make a class and create an instance of the object...why do some people use :: and some use ->

so , for example in my book they go:

CourseFactory::getCoursesForStudents($objStudent), where i would go

CourseFactory->getCoursesForStudents($objStudent)

t5hanks!
 
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