I am not very familiar with mainframes, except I know they are much
larger than a PC.  So my questions is, what is the difference between
Mainframes and PC's.  Are mainframes really necessary or can today's
PC's replace all mainframes?

Hello Rech17,

The main difference between a mainframe and a personal computer is a
difference in emphasis. Both have...
 - one or more central processor units
 - memory (may be multiple units on a mainframe)
 - one or more busses
 - one or more I/O systems
A personal computer will generally have only one of each. A mainframe
can have several and can often be divided into several (even
thousands!) logical partitions to perform a number of operations
concurrently. A mainframe may have a unit specially designed to manage
the entire system. A mainframe will often have far more I/O
performance than a personal computer as well.

To answer the question "Are mainframes really necessary?" requires
some background similar to that provided by other comments. If you
have an application that requires a lot of I/O performance (e.g.,
serving static web pages) - you have at least two alternatives:
 - a mainframe
 - a set of smaller servers (e.g., PC) with load balancing and other
software not related to the application

Both can do the task, but the the first solution may be quite a lot
easier to implement than the second one.

As another person has commented - a company may have a lot of older
software to run on mainframes. It may be more cost effective to
continue to run that software than to convert it to run on new (lower
cost) equipment.

So, you find that mainframes will continue to be used for some time.
For some additional references, you can look at...

Running Linux on mainframes
http://www.linuxworld.com/site-stories/2002/0416.mainframelinux-p7.html
http://librenix.com/?inode=51

An online answer to a similar question
http://www.experts-exchange.com/Hardware/Q_20190791.html

A suggestion on search phrases for mainframe to PC comparisons
http://search390.techtarget.com/ateQuestionNResponse/0,289625,sid10_cid487945_tax285123,00.html

--Maniac

This is a brief comment.
Main frames are the computer machines, has high power, able to handle
multiple users (hundreds or thousands) concurrently and able to handle
complex calculations.

Mainframes are the only commercially available platform to automate
the business applications (most of data handled is characters and
numbers- No pictures) of big company's during 1960 to late 80s.
Mainframes are cost intensive.
You can learn more about this in the following link
http://www.mainframes.com/whatis.htm

PCs started as low-end single user computing machines during early
80's. But over the period, with the advent of high power computer
processors the power and design of PCs grown very fast. With the use
of LAN/WAN network and multiple processors (Normally used in Servers)
they able to serve multiple users/complex calculation tasks also. They
can handle data combination of pictures, character and numbers sound
etc.
Due to Mass production PCs is available at very low cost.
The following link gives the timeline of PCs starting from the
invention of
Transistor. 
http://www.fortunecity.com/marina/reach/435/


Now pcs which are much powerful than mainframe are readily avilable at
much lower cost than the mainframe. So PCs can technically replace all
the tasks(except very few complex applications like Weather
Forecasting).

Then why mainframes are still used by big corporates?
The reason is  who bought mainframes had invested a huge sum on
computer assets.
Over the period they build up  huge code base(million or billion lines
of code).
It is very  capital intensive and time consuming to replace all
mainframe applications to a new PC platform. This for the same reason
why companies spent huge sum to make their old mainframe code to
handle year 2000.

So the best solution followed by the companies is
Develop new applications using wholly on PC platform or using PC
platform and with connectivity to existing mainframe.

Mainframes are still necessary, and can even be cheaper than PC based
servers. There's a useful summary from the ibm mainframes newsgroup at

http://os390-mvs.hypermart.net/mainpc.htm

There's a summary of some comparative costings at

http://consultingtimes.com/Serverheist.html

Owain