Mainframe computers are typically:
- Powerful - they can process vast amounts of data, very quickly
- Large - they are often kept in special, air-conditioned rooms
- Multi-user - they allow several users (sometimes hundreds) to use the computer at the same time, connected via remote terminals (screens and keyboards)
Personal Computer (PC)
These computers came to be known as desktop computers, or personal computers (PCs).
A typical PC contained the same basic components as a mainframe computer (CPU, RAM, storage, etc.) but at a fraction of the size and cost.
Laptops contain a rechargeable battery so that they can be used even when not plugged in to a mains power supply. They also have a built-in LCD monitor.
To make them as portable as possible, most laptops try to avoid any sort of cable or wire. Instead of a mouse, a trackpad is used. Instead of a wired connection to a network or printer, 'wireless' radio connections are used.
Palmtops are usually not very powerful since fast CPUs require a large battery and get hot - both problems in a small device.
A typical palmtop have a very small keyboard - too small to type on normally. Instead the user types using both thumbs. Also there is no room for a trackpad, so a touchscreen or tiny joystick is used instead.
Palmtops are extremely portable, but the small keyboard and screen make the devices tiring to use for long periods.
Personal Digital Assistant (PDA)
Most PDAs use some sort of handwriting-recognition system to allow the user to write on the screen, and have their writing converted into text.
PDAs tend to be used a 'digital diaries' allowing users to take their e-mail, documents, appointments, etc. with them wherever they go.
Note: You never see PDAs any more since modern 'smart' phones can do all of this, and work as a phone too!