What Are They For?
Twitter Lists allow you to group together the users you are interested in, in a manner which suits you. The idea behind the List function is that it lets you view a feed of tweets only from those whom you have added to a list, meaning that you effectively filter out everyone else when you view that list. This has the benefit of helping you to more easily keep track of tweets on a particular topic of interest. For example, if you followed 100 people and 40 of them were lawyers, you could create a list for those 40 lawyers and when you viewed that list, only their tweets would appear.
How Do I Set Up A List?
This is really easy. On your homepage, on the right hand menu, there will be a Lists drop-down menu (below the Search box). Simply click on New List, and then give the list a name (whatever you like) and an optional description of up to 100 characters. You can then choose whether to make your list viewable by others on Twitter by leaving it set to Public, or keep the contents of the list to yourself by making it Private. There is probably more to be gained by setting your lists to Public - this could encourage those who are included on your list to follow/list you back, and it also potentially helps those users to obtain more followers because their tweets and profile will be viewable by those who come across your list.
How Do I Add Users To My List?
Once you have set up a list, you will be given the option to search for users to add at that point. You can either do it then or simply by clicking on the List icon, which you will tend to find alongside a user's name and elsewhere, and which looks like this:
Note: You don't have to follow someone to add them to your list, or to see their tweets appearing on someone else's list that you follow.
How Do I Subscribe To Other Users' Lists?
If you see a list you would like to subscribe to and which is not set as Private (this would be marked with a padlock icon beside it), click on it to display it and then simply click "Follow This List" under the heading.
How Do I Know If I Am On Other Users' Lists?
Alongside the counts on your profile page showing the number of your followers and those you are following, you should see the count representing the number of lists which you have been included on. Click on this number to view these lists.
How Do I Mention A User's List In A Tweet?
If you want to mention a list in a tweet, simply type in the username of the user who created the list, followed by a forward slash, followed by the list name. For example, if you wanted to mention David Flint's list of MacRoberts' Twitter users, called "macroberts-tweeters", you would type it in as follows: