What is the difference between RS 232 and Ethernet cable? What will happen if one is replaced by another??

RS232 (serial) is NOT interchangeable with RJ45 (ethernet)

RS232 tends to be used for comms, such as hyperterminal, but has mostly been superseded by RJ45 and the TCP/IP protocol.
More information can be found here:
RS-232 - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Rs232)
AND
8P8C - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/RJ45)

Curlyben gave you the 100% correct answer. Instead of the Geeklish version, the plain English version is that RS-232 serial communications are SLOW and the Ethernet connections over RJ-45 cables are hundreds of times faster. I would choose fast whenever possible.

Thanks for that chuckhole, must remember to use English in future ;)

Curlyben... See PM... :)

RS232 and Ethernet can both use CAT-4, CAT-5 or CAT- 6 cable. RS 232 probably can get away with CAT-3. The difference is in how well and uniform the twists are made in the cable.
RS232 usually doesn't care about the twists. RS-422 and RS48 will. Ethernet speed is dependent on cable and connections. Thus, CAT-4 can't be used on gigabit Ethernet.