The first obvious difference between a PC and a Mainframe is their size or "footprint". A pc is about 18" square and maybe 8" wide. Most people could take one apart with little or no training and install a new component like a modem or sound card. You can go buy a PC at most computer stores for less than $1500 and have a relatively powerful machine. In contrast Mainframes are very expensive, large in size, and your average Joe would not be able to open one up and simply add new components. They are usually installed in temperature-controlled rooms with access being limited to authorized personnel. They are not portable, at all. Maintenance is usually carried out by a third-party under strict, multi-million dollar contracts. A typical PC will have one or two hard drives, with 40 Gigabytes of hard drive space each, and 256 to 512 MB of RAM.
Mainframes on the other hand, can come with hundreds of hard drives in a sophisticated RAID array with 30+ Terabytes of storage capacity and
typically Mainframes will have Gigabytes of memory. Most PCs will run Windows and function as Clients in a network environment while Mainframes exist only as Servers and typically run some form of UNIX/Linux.
There are a lot of variables when comparing the differences between PC's and Mainframes, some of which include performance, reliability, and cost. In terms of raw CPU speed, there is probably not as big a difference as their once was. The differences lie mainly in reliability and I/O capabilities. Mainframes have a much larger I/O bandwidth than PC's have. In addition Mainframe's have a lot of redundancy and serviceability features built in. The way they handle I/O is also much more efficient than the 'lower end' PC boxes. In addition, Instead of...