A mainframe is a giant, powerful computer hooked up to a network that serves as a primary computation point for large distributed systems. Usually used by businesses.
Some words that might come to mind that are related to this word are supercomputer or server farm.
These are rarely interacted with using a standalone keyboard / mouse / screen sort of thing. Instead, they are logged into from other computers called workstations.
LOTS of processing power, RAM, network bandwidth. An OS designed to handle lots of distributed processes from many different sources at the same time.
Honestly, the word server is just used for a smaller mainframe. See everything up above.
Main resource: networking bandwidth, some might have lots of processing power depending on the need
## PCs / laptops / sub-laptops
All of these are basically serving the same purpose - they are smaller personal devices that are primarily used by one person, at least a time, with their own peripherals and input interface.
Sure it has its own processor power and networking bandwidth, but only enough for one person. It also has to manage peripherals and screen resolution and things like that, which a server does not need to bother with, generally.
PCs of course vary widely in power, with big fancy desktops being up to 10x as powerful as small little sub-laptops.
To consider in context: size, portability, battery power, power usage, etc.
PDA - Personal Digital Assistant. These basically don’t exist any more! They were standalone devices that could be held in your hand and ran productivy apps like calendars, contact lists, etc. They eventually merged with phones and created the modern smartphone.
Phones - You know what a phone is. It’s your life. Get off your damn phone.
Lower processing power, smaller screens, limited input options. Much less energy, small batter means they might not be usable for hours straight. Extremely portable. Cameras.