Full Access is exactly what the name implies: the user has access to all the profiles for the entity type. This is the default setting for all agents.
No Access access means that the user cannot access any of the profiles for this entity type. If you have one entity type and you block the agent, that agent will not be able to access any user profile. But suppose you have two entity types (for example, users and employees). If you block the agent from the user entity type, he or she will still be able to access profiles that use the employees entity. To prevent the agent from accessing any user profile, you will need to block the agent from both users and employees.
The Limited Access filter is explained in more detail in the article Restricting Agent Activity by Profile Type.