What exactly is a "role" in Capistrano?

Roles allow you to write capistrano tasks that only apply to certain servers. This really only applies to multi-server deployments. The default roles of "app", "web", and "db" are also used internally, so their presence is not optional (AFAIK)

In the sample you provided, there is no functional difference.

The ":primary => true" is an attribute that allows for further granularity in specifying servers in custom tasks.

Here is an example of role specification in a task definition:

task :migrate, :roles => :db, :only => { :primary => true } do
  # ...
end

See the capistrano website @ https://github.com/capistrano/capistrano/wiki/2.x-DSL-Configuration-Roles-Role for a more extensive explanation.

The ":primary => true" option indicates that the database server is primary server. This is important for when you want to use replication with MySQL, for example. It allows you to create another mirrored database server that can be used for automatic failover. It's also used for deciding on which database server the model migrations should be run (as those changes will be replicated to the failover servers). This link clarifies it a bit more: https://github.com/capistrano/capistrano/wiki/2.x-from-the-beginning#back-to-configuration