Autofollow - Automatically following other users who follow you. This is a setting in Twitter, but tools which integrate with Twitter may also automatically follow other users.

Block – If you don’t want to ever see anything another Twitter user says, you can block them.

Direct Message/DM - Private messages sent to other Twitter users who are following you, and whom you follow.

Follow/Followers/Following – When you want to read what another Twitter user posts, you follow that user. Your followers are all the people who are interested in what you have to say

Hashtags – Hashtags are a word prefaced with a “#” sign. This is used for searching popular topics

OH - An abbreviation for “overheard."

Reply - A tweet that is a public message to another Twitter user. These tweets begin with @.

Retweet/RT – Copying another Twitter user’s tweet and adding the letters “RT” plus their Twitter username.

Trending Topics - popular #hashtags are tracked by Twitter. The more people that use the #hashtag, the high the rating.

Tweeple/Tweeps – Tweeple is a combination of twitter and people, meaning twitter followers. Tweeps is a similar combination of twitter and peeps.

Tweet/Tweeting – What you call a post on Twitter. Tweeting is what you’re doing when you’re writing posts on Twitter.

Twit – nickname for a Twitter user..

URL Shortener – A URL is a web page address. Because of the character limit in Twitter, long URLs are not useful. A URL shortener service redirects your long URL to a much shorter one.Twitter does this automatically.

Other Twitter 101 Sources:

How do I set up a Twitter Account

What is #followfriday on Twitter?

How do I choose a username on Twitter?

Twitter Followers: Should you pay for them?

How can I use Twitter to find a job?