Many times both can be very clear and sometimes it may look confusing or overlapping. In IaaS, you got just infrastructure where you can run your OS, DB, all other stuff. You install it, manage it and all. It is all your headache. In Platform-as-a-service, you get OS, DB and all other stuff in a bundle. You have choice in different bundles you choose. Different bundles means different combination of OSes, DB and others. Service provider will install and manage them all. You do not need to bother to install and configure at all (Linux, PHP, MySql,....).