Twitter defines a list as "a curated group of Twitter accounts." You could say it is a list you create of Twitter users that have similar hobbies, or who post about a particular topic that interest you, or are your college buddies. The possibilities are limitless.
Lists can be public or private; one may subscribe to other's lists, and you may even share the link to a particular list. For the basics on creating a list, consult the Twitter Lists page here:
If you are new to social media marketing, my recommendation is to start by creating a list of other social media marketers. You can learn so much from other experts in the field as they can give you ideas, keep you updated on the SMM trends, and you can even interact with them by beginning a conversation when you reply to their tweets.
Below is a small collection of articles on the topic of lists to get you started:
Today’s Smartest Baby Boomers Use Twitter Lists
The Best Ways to Organize People You’re Following Into Twitter Lists
Why you definitely should be using Twitter Lists
I have a couple of Social Media Marketers lists in Twitter and I invite you to peruse them.