Lets be clear...
There is 3 main components on an SAP system that you can split up.. SAP Central Services, Database & Application server.
High availability is provided in different ways to each of them.
In a regular scenario central services use an enque replication server, Database is usually replicated/mirrored using log shipping to a secondary database and at application server level simply use logon balancing to multiple hosts to achieve HA.
In top of that you have automatic failover (cluster) which can be used for both SCS and DB.
Basically a distributed system is one were the components have been installed in different hosts.
Regards, Juan