All definitions are directly quoted from the The Twitter Glossary found at the Twitter Help Center.

Direct Message: Also called a DM and most recently called simply a "message," these Tweets are private between the sender and recipient. Tweets sent over SMS become DMs when they begin with "d username" to specify who the message is for.

Favorite: To favorite a Tweet means to mark it as one of your favorites by clicking the yellow star next to the message. You can also favorite via SMS.

Follow: To follow someone on Twitter is to subscribe to their Tweets or updates on the site.

Follower: A follower is another Twitter user who has followed you.

Following: Your following number reflects the quantity of other Twitter users you have chosen to follow on the site.

Handle: A user's "Twitter handle" is the username they have selected and the accompanying URL, like so:

Hashtag: The # symbol is used to mark keywords or topics in a Tweet. It was created organically by Twitter users.

Mention: Mentioning another user in your Tweet by including the @ sign followed directly by their username is called a "mention". Also refers to Tweets in which your username was included.

@: The @ sign is used to call out usernames in Tweets, like this: Hello @Twitter! When a username is preceded by the @ sign, it becomes a link to a Twitter profile. See also Replies and Mentions.

Profile: A Twitter page displaying information about a user, as well as all the Tweets they have posted from their account.

Protected/Private Accounts: Twitter accounts are public by default. Choosing to protect your account means that your Tweets will only be seen by approved followers and will not appear in search.

Reply: A Tweet posted in reply to another user's message, usually posted by clicking the "reply" button next to their Tweet in your timeline. Always begins with @username.

Retweet (noun): A Tweet by another user, forwarded to you by someone you follow. Often used to spread news or share valuable findings on Twitter.

Retweet (verb): To retweet, retweeting, retweeted. The act of forwarding another user's Tweet to all of your followers.

RT: Abbreviated version of "retweet." Placed before the retweeted text when users manually retweet a message. See also Retweet.

Timeline: A real-time list of Tweets on Twitter.

Tweet (verb): Tweet, tweeting, tweeted. The act of posting a message, often called a
"Tweet", on Twitter.

Tweet (noun): A message posted via Twitter containing 140 characters or fewer.

Twitter: An information network made up of 140-character messages from all over the world.

URL: A Uniform Resource Locator (URL) is a web address that points to a unique page on the internet.