Mailing Lists are a way to formally group Contacts together that you intend to email regularly so that you can send them specific and targeted emails. Example uses of Mailing Lists
By uploading Contacts into a Mailing List (done Step 3 of the Import Contacts Workflow), they will be added to any existing Contacts you have already imported to that List. Just keep adding them to the pot and Sentori will look after these segments for you.
You do not need to use Mailing Lists; it is perfectly OK to just upload your Contacts into your Account without doing any kind of segmentation. In this case, you would just choose to send to Everyone.