Organization is the highest element in the hierarchy. An Organization typically represents your Company where-in you setup all your Users who will have access to your data. You can setup different working areas (a.k.a.) Workspaces to separate data into different access zones. A Workspace is flexible by design so you can create Workspace to represent each Department within your Company or use them to manage different business solutions like one for your Sales and Marketing operations, another workspace to manage your Projects with Clients etc.
A Workspace, created under an Organization, is accessible to those members who are part of the Workspace. It has a set of Application(s) where your actual data resides and is accessible to users that is governed by access rules you setup as an administrator. You can create as many Workspaces under the Organization as you like.
Application come under Workspace and contain your data that could be anything from your CRM Contacts, Customers, Sales Deals to any kind of custom data you would like to store and manage. Fusionmint Applications come in two flavours, Standard and Custom apps. Standard apps. that are pre-made by Fusionmint come with advance features for different business solutions such as Sales, Marketing, Project Management and more. Custom apps. are the ones that you can create yourself suitable to your business needs, this could anything from your Inventory, HR Leave Applications or just about anything you can imagine.
Domain exist under Workspace and govern the kind of access you would like to give to different users in Workspace. In a Domain you can setup access to all or a limited set of Applications, decided whether users have access to Create and/or Edit records or just View only. They provide a far greater level of access control and let you hide confidential information such as Costing, Budgeting or any other private information that you would not like to share with your users. When it comes to collaboration with internal team and external partners & vendors, Domains provide you a very robust and rich set of access control mechanism to secure your data.
Role is a collection of users and is meant to group users with similar work and access capacity. For e.g. if you are working with an external partner team with 5 members, you can create a role for them and add those 5 users to one Role and attach it to a Domain. Then, all those 5 users will have same access to Workspace as decided by you in the Domain setup.
User is anybody who has an account with Fusionmint. They can have access to any Organization and Workspace if they have been given access by the Administrators.
More details on each term are available here