Due to the 140 character limitation to tweets users have adopted a lot of acronyms to convey meaning without wasting precious character real estate. Some of these originated on Twitter, while others have been in use for far longer. Below I've gathered together some of the most common acronyms and their meaning and use.
RT: Retweet. Retweeting is sharing a tweet in its entirety with your followers. You can retweet the tweet "as is" using Twitter's Retweet button or you can manually retweet a tweet by copy-pasting a tweet and placing RT before the @handle. Note though that the latter is usually only done if you mean to add a comment on the tweet. Here are two examples of both sorts of retweets from the Leiden University Twitter account.
MT: Modified Tweet. A modified tweet is a retweet of someone else's tweet, but one that has been changed to allow for a comment.
h/t: Hat tip, used to give acknowledgement to the person who pointed you in the direction of the article, video or other information you're sharing. Sometimes this is done with the prase via @handle at the end of a tweet.
ICYMI: In Case You Missed It. Mostly used when re-sharing a previously tweeted link.
DM: Direct Message. Direct messaging allows you to contact someone directly and in private.
@Pallekenl Check your DM!!— Liz de Jager (@LizUK) October 31, 2013
OH: Overheard. For those times you want to share something interesting overheard offline:
OH in the library..."I didn't like that book. It had too many words!" - 1st grader— Stacy Dillon (@mytweendom) December 9, 2013
^: The caret or hat sign is used to denote that a tweet was made by a particular member of a team responsible for tweeting from an account. In the tweet below, for example, the ^M shows that I was the one who sent this tweet from the @UBLeiden account.
Nu permanent in dit theater: Asian Film Online. Bijna 500 Aziatische films en docu's over diverse onderwerpen: http://t.co/abhiiJklcs ^M— UBLeiden (@ubleiden) April 24, 2014
If you'd like a more extensive list of common Twitter and or internet acronyms you can find some at the following links: