When you log in to Twitter, you'll land on the Tweets timeline view of your homepage, as shown in the image below.
Your home timeline is a long stream showing all Tweets from those you have chosen to follow on Twitter.
The newest updates are at the top. You can interact with Tweets from within the timeline by hovering your mouse over a Tweet to reply, retweet, or favorite.
Clicking anywhere on a Tweet in your timeline expands the Tweet, so you can see photos, videos, and other information related to that Tweet.
Note: If you see Tweets in your timeline from users you don't follow, check to see if they are retweets. It's likely someone you do follow chose to re-broadcast a message because they found it interesting. Retweets will show up in your timeline, unless you have them turned off for a particular user.
Where you'll see other timelines:
Timelines can also consist of collected messages from users in lists that you've curated or as search results.
When you click on a list, you will see an aggregated stream of Tweets (a timeline) posted by the users included in that list.
Similarly, when you perform a search, you'll see a timeline of messages that all match your search terms.