|what is the use of Static IP|
Typically, static IP's are used for such things as servers, printers and network appliances who's IP's should never change.
can i use this for shareing Files, Printers etc over internet
Yes you can, I do that all the time. I have a somewhat complicated setup at home (comes from being a professional computer geek for a long time) but I won't get into the complex part, I'll give you a more "typical" setup.
Internet >> SOHO Router >> Clients
Now let's say your using the following subnet in your LAN: 192.168.0.0/24
with the following DHCP Scope defined:
192.168.0.100 to 192.168.0.199
Now, you have a Windows based computer in your LAN that you store files one and wish to be able to access remotely. Because it's going to be providing a "service" you will want a static IP address so you assign it one outside the DHCP Scope. So for this example we'll give your Windows computer: 192.168.0.10
To access it remotely you would first allow Remote Desktop connections. You do this in your System Properties on the "Remote" tab (System Properties can be found in Control Panel).
Then you would create what is called a "Port Forward" on your SOHO Router. RDC (Remote Desktop Connection) uses port 3389 so on your router you would forward port 3389 to the IP of the PC you wish to connect to. Using our example above your forward would look as follows:
Forward Port 3389 to 192.168.0.10
Now, all you need to know to connect remotely is the public IP address of your internet connection. The from wherever, you would use the RDC client to connect to that IP and when you hit the firewall, it will automatically forward your connection to the PC you specified in the port forward.