Special sessions are introductory overview lectures or late-breaking sessions presented concurrently with other sessions. More than likely, these will be given by renowned statisticians.
Invited sessions, including plenary sessions and named lectures, are pre-organized sessions with speakers who have been invited or are expected to give talks based on their position in the organization (e.g., the President's Invited Address). Plenary sessions are given at a time during which they do not compete with other sessions. Awards are given at some of these events, as well (e.g., Sunday and Tuesday nights' invited addresses).
Topic-contributed sessions are planned about eight months in advance of JSM. These are typically based on a single topic and organized by one or two people.
If you submitted an abstract to the JSM planning committee, you will most likely be in a contributed session. These are papers that have been grouped together based on a theme.
TIP: The type of session will not necessarily reflect its quality. In the past, promising talks have fallen short due to an apparent lack of preparation, while some contributed session presentations were excellent, as it was obvious they were rehearsed. Go to sessions you think will be interesting; do not limit yourself to only "renowned" speakers.