When contacts are linked to Actions (matters) they are referred to as Action Participants or Parties.
Each Party is of a certain type (client, defendant, attorney, etc) and the same contact may be linked to the same Action several times as different participant types (for example someone may be the client and defendant in the same Action).
"Action Participants" and "Parties" refer to the same thing and the terminology can be used interchangeably.
The same applies to "Participant Types" and "Roles".
The Primary Participant of a matter is useful as it displays that participant's contact information at the top-right of the matter.
It can be identified in the 'Parties' list by looking for the star next to the participant type.
The Primary Participant can also be used to search for Matters in Time Records.