when I'm trying to read document about new system structure i have some problem to understand the servers requirements like the different between clustered, redundant, failover ?
This is a bit vague, but for the three terms you gave:
Clustered: This means, at a basic level, that two or more servers are combined together to act as one in some way. This may involve a computing cluster, such as in "grid" computing, where all the cluster nodes are computing a portion of the workload in order to scale the power of the system. Alternatively (and more commonly), it may involve a failover cluster of nodes that are designed to take over for each other in the event that one (or more) fails. In that case, a particular service only runs on a single node at a time, but may be moved to another when needed.
Redundant: Redundant nodes are those in a failover cluster, where they are designed to take over operations for one another in the event of the failure of one of the nodes.
Failover: The term for when services on one node "fail over" to another node in the event of an emergency, or when manually moved.
In simplistic terms, this is what those terms mean to me.