- What are mounted servers and are there any other types of servers too?
- What are different configuration? I have heard about RAID. Is this the way multiple servers are connected to form a single server cluster? What are other options?
A mounted server is not a type of server.
In the context of your question a mounted server is likely a server who's disk or filesystem is accessible and in use by another server - for example it would be "correct enough" to say" NFS server X is mounted on server Y, where you most likely mean the filesystem of server X can be accessed by server Y.
RAID stands for "redundant array if inexpensive disks", and means exactly that - taking multiple disks and making them appear as a single large disk. There are different types of RAID depending on what combination if speed, reliability and size of disk you want. Importantly, RAID describes disk layout, not the way the disks are exposed to other machines. It exists at a block level (is like a raw disk or partition - something you can then put a filesystem on, where the filesystem just thinks its on a disk or partition)
Both of your questions appear to be conflating disk/storage with servers. While servers can certainly be dedicated to supply disk resources this is not normally the primary role if them.