Twitter timelines are list of tweets or messages displayed in the order in which they were sent, with the most recent on top.
There are different types of Twitter timelines. Home timelines are what every Twitter user sees on their home page by default--a list or stream of tweets from all the people they follow, which gets updated in real time.
There are also timelines generated by Twitter lists. These Twitter timelines display messages from users included in a list you follow; they can be lists of users that were created by you or lists created by other people.
Search results also form Twitter timelines. They show messages matching your search query in a chronological list.
This Twitter timeline tutorial explains more about how a basic timeline works. It also provides a list of third-party tools for making better use of Twitter timelines.
Interacting with Twitter Timelines
The main thing to know is that you can interact with each message in timelines simply by clicking on the tweet; it will expand to show you any media associated with it, like a video or photo; or who's replied or retweeted it; or any other relevant Twitter conversations relating to that particular tweet.
Twitter timelines have changed a bunch of times as Twitter updates its user interface, so don't be surprised if the list of your tweets changes in appearance from time to time. Twitter continues to experiment with ways to display sponsored tweets and ads in or near timelines, so that's one area to watch.
This Twitter language guide offers additional definitions of Twitter terms that can baffle new users of the micro-messaging service.