If you asked this question, it is an indication that you are a newbie entering the well etablished world of Social Networking. Twitter is essentially a Social Networks tool and a good one at that with phenomenal growth from the time they were founded in 2006.
The traditional Dictionary will you that the word Twitter means the high pitch sounds originating from birds, usually as a continous series of chirps. Nowadays a reference to Twitter is synonymous with the Social Network “Twitter”. Thankfully birds are not forgotten as Twitter (which hereinafter will mean the social network Twitter unless otherwise stated) uses birds as their mascot and symbol, not a bad choice for a metaphor. For example it becomes cute when we see the word “tweet” is used by Twitter to show the messages sent and received within the network!