PaaS (Platform as a service) is a cloud model in which computing platform is offered as a service rather than equipment. This means that an OS (Operating system), Database, language execution environment or/and a web server is offered over a network environment (Typically, the internet).
SaaS (Software as a service) is a cloud model in which software is remotely offered as a service. This means that rather than installing a typical software onto your system, you can access it and use it using a web browser or a cloud-based application.
IaaS (Infrastructure as a service) is a cloud model in which computer itself is delivered as a service. This means that Cloud service providers can offer you virtual machines, networks, firewalls and raw storage blocks. IaaS providers usually bill the end user based on utility computing basis (amount of resources utilized).
Service providers may offer access to their cloud via Laptops, tablets, smartphones and desktops. Now, some of the cloud service providers are offering their services via thin clients and the Chrome-book.