Difference between parallel and distributed database?

Centralized Systems: A modern, general-purpose computer system consists of one to a few CPUs & a number of device controllers that are connected through a common bus that provides access to shared memory. The CPUs have local cache memories that store local copies of parts of the memory, to speed up access to data. Each device controller is in charge of a specific type of device (for example, a disk drive, an audio device, or a video display). The CPUs & the device controllers can execute concurrently, competing for memory access. Cache memory reduces the contention for memory access, since it reduces the number of times that the CPU needs to access shared memory.
In a distributed database system, the database is stored on several computers. The computers in a distributed system communicate with one another through various communication media, such as high-speed networks or telephone lines. They do not share main memory or disks. The computers in a distributed system may vary in size & function, ranging from workstations up to mainframe systems.
The main difference between centralized & distributed databases is that the distributed databases are typically geographically separated, are separately administered, & have slower interconnection. Also in distributed databases we differentiate between local & global transactions. A local transaction is one that accesses data only from sites where the transaction originated. A global transaction, on the other hand, is one that either accesses data in a site different from the one at which the transaction was initiated, or accessed data in several different sites.