Direct Message (DM) -- a private message sent to one of your followers via Twitter; you can only send a direct message to one of your followers, you can only receive a direct message from a user you follow

Follow/Follower/Followers/Following -- to follow (v) a user on Twitter is to ‘opt-in’ to receiving their Tweets, subscribing to their Twitter account; doing that makes you a follower (n) of that user, just as someone who ‘subscribes’ to your Tweets is your follower (n); your followers number is how many users who have subscribed to receiving your Tweets, your following number is how many users you have opted-in to receiving their Tweets

Hashtag -- the # symbol, used to mark keywords/topics in a Tweet; placed before the relevant keyword/phrase (no spaces), serves as a means to categorize messages, makes searching easier; can be placed anywhere in a Tweet; best practices recommend a maximum of two hashtags per Tweet and that hashtags are used only on relevant Tweets

Reply -- it is what you think it is – posting a response to a user’s Tweet; always begins with @username where username is the person you are replying to; done by clicking the ‘reply’ option on their Tweet in your timeline

Retweet (RT) -- (n) a Tweet from another user you repost/republish on your account; used as a means of sharing news, interesting content, etc. from another user to your followers; identified with ‘RT’ to show the Tweet is from another user and that you are sharing/passing it along
(v) the act of sharing another’s Tweet with your followers, reposting that Tweet to your account

Trends/Trending -- what’s registering as most popular on Twitter at a particular moment; determined algorithmically by Twitter