Programming :: Filter Out The Additional Packages That Exist On A New Server?

I've got two files. They both contain package names. Is there any way I can go through the package list on one file, and search to see if each package exists in the other file? What I'd want to do, is if the package name is found in the the main file, then go to the next line. If its not found then print that package name to another file.

I know you can use diff, but it doesn't seem to be that straight forward. As I understand it diff searches line by line, so if line one doesn't match line one in another file, then it prints it out. That's not exactly what I want.

I just really need an easy way to filter out the additional packages that exist on a new server. If I have a list of packages that aren't on the original server, then i can just delete them.

Not sure if I've made any sense but there must be a quicker way to do what i need to. It would take me ages to scan manually through the package names in each list, and highlight the ones i dont need.