A retweet is a simple way to re-post someone else's Tweet. By retweeting, you can share another user's content to your followers and the Twitter community at large.
There is no limit for the number of retweets a single Tweet can have, however, Twitter will only display the most recent 100 retweets. Also, retweets are counted against a user's 1,000 Tweet per day limit.
According to Twitter.com −
Retweets look like normal Tweets with the author's name and username next to it, but are distinguished by the Retweet icon and the name of the user who retweeted the Tweet. If you see a message from a stranger in your timeline, look for retweeted by info in the Tweet — the retweeter should be someone you follow
Sometimes people type RT at the beginning of a Tweet to indicate that they are reposting someone else's content. This isn't an official Twitter command or feature, but signifies that they are quoting another user's Tweet.
Retweets may appear differently in third-party applications, and will show up in apps only if they are using Twitter's Retweet API. Many apps have built in their own version of retweeting — those Tweets are not treated as official Retweets on Twitter. To test, try retweeting from your favorite app, then check your profile on the web.
Here is where you can get to see the different tweets −
You can see Retweets your followers have retweeted in your Home timeline.
Retweets, like regular Tweets, will not show up from people you've blocked.
If you retweet someone you follow, it will be visible on your profile timeline.
If you retweet someone you do not follow, it will be visible on both your home and profile timelines.
Go to Notifications in the top navigation bar. In the Notifications section you will see all activity concerning your Tweets — including which have recently been retweeted and by whom
You can turn off Retweets for a specific user if you don’t like what they share.
Select Turn off Retweets from the gear icon drop-down menu on a user's profile.
Turning Retweets on or off is not retroactive; means you can't remove Retweets that are already in your timeline.
It is not possible to turn off ALL Retweets from ALL users.
You can undo a Retweet you've made by clicking on Retweeted in the Tweet. This will remove the retweet from your timeline, but will not delete the original Tweet.
Here are some reasons why some tweets can’t be retweeted −
If another user's Tweets are protected, you will not be able to retweet their content. If you see the Lock icon next to the user's name and information on their profile page or on their Tweets, their Tweets are protected and you will not be able to share their Tweets on your timeline through Twitter's Retweet feature.
You can see their Tweets in your timeline because they have accepted your follow request, but because they have chosen not to share their Tweets publicly, their Tweets cannot be retweeted by you or anyone else.
You cannot retweet your own Tweet. However, if your Tweets are public, others will be able to retweet you.