Digital:

Relating to or operating with signals or information represented by discrete numeric values of a physical quantity such as voltage or magnetic polarization (commonly representing the digits 0 and 1); designating a signal or information of this kind. Opposed to analogue.

http://dictionary.oed.com.libraryproxy.griffith.edu.au/cgi/entry/50063921?single=1&query_type=word&queryword=digital&first=1&max_to_show=10

Analogue:

An analog computer is a form of computer that uses the continuously-changeable aspects of physical phenomena such as electrical, mechanical, or hydraulic quantities tomodel the problem being solved. In contrast, digital computers represent varying quantities incrementally, as their numerical values change.

http://en.wikipedia.org/wiki/Analog_computer

Communication:

Communication is any process that transfers, transmits or makes information known to other people.

from lecture notes

Electronic:

When comes to a device, electronic means operating according to the principles or methods of electronics.

http://dictionary.oed.com.libraryproxy.griffith.edu.au/cgi/entry/50073038?single=1&query_type=word&queryword=electronic&first=1&max_to_show=10

Examples:

1.Flag semaphore

http://www.thebeatles.com/#/albums/Help2

2.Abacus

http://archive.computerhistory.org/resources/still-image/abacus/abacus.a.102622629.lg.jpg

3.Braille

http://tokyo5.files.wordpress.com/2010/04/braille-fares.jpg