I cannot make out the DIFFERENCE between Ethernet and my USb modem.
When I google for any networking help, most of the websites gives me info about Ethernet but I can HARDLY get any help about my usb modem.What is the difference between these two??
What problems are you having?
I am having usual problems from TIME to time such as USB is not connecting or some others.
At this moment I do not have any problem but wanted to know the difference only for knowledge and to TACKLE the problems  which may arise in futureEthernet is how computers connect to each other over a network. It is an older, very stable connection method.

USB (Universal Serial Bus) is newer and more traditionally used for connecting hardware such as printers, keyboards, mice, cameras, and so on. It can be less stable than ethernet but its advantage is that in the most basic of ADSL modems, you do not require a separate power cord.

Most people tend to recommend ethernet modems for connecting as it is less temperamental and easier to install, which makes connection troubleshooting much simpler. Ethernet modem/routers operate independently of your computer and functionality doesn't depend on which OS you run on your computer.

Google for "USB vs Ethernet modem" - you will find much information.
Problems with USB are sometimes reported.
USB is a protocol that differs from Ethernet. So that can make a higher level of complexity. So some administrators decide to avoid USB when possible.
Some examples: Mass storage HDD that CONNECTS via Ethernet.  Printer that connects through Ethernet. FAX and Scanner that works with Ethernet.