What's the difference between a common mobile phone, a smartphone and a PDA?

Answer 1

Any phone that can make and receive calls using a cellular signal is a mobile phone
A PDA is not necessarily a phone and offers functionality to schedule appointments, reminders and take memos.
A smartphone is a mobile phone that has an installed operating system and allows users to install third party applications on it

A smartphone usually has PDA functionality but the reverse is not necessarily true

Answer 2

2

Answer 3

Common cell phone has only 1 primary functions, perhaps 2; and the rest are subpar after-thoughts addons; and the nature of its OS restricts expansion.

A smartphone has multiple primary functions; usually well-defined and restricted to what they are but subpar addons can be expanded on a need basis.

A PDA is fully expandable in its design.

Microsoft would have you believe a Smartphone (not smartphone) and a Phone Edition PDA differs only in size and touchscreen/less input, perhaps that's the answer you want, but a common cell phone always is inferior to others in its own time period.