as an IT Staff b4 main work nako is just troubleshooting, fixing computers and assisting.. all the basic computer theories and imo skill mapuga jud dre nga part mao ni pinakahasol sa tanan esp gubaon/old pc models nya d pa jud ipaupgrade sa imo boss kay magamit pa daw(it's like hell nindot au patiran ang PC ahahahaha).. but the good thing is you'll learn and once you learn you are a step behind to the upper position..
Network Administrator - is responsible mostly sa security sa network ninyo, giving different types of access to different types of work to the clients/user and other Networking stuffs using the servers(well it depends if unsa imo job description sa company)..
Sytems Admin - mao ni dre ang mga programming stuffs(SQL, mySQL, etc..) nga mumanage sa mga domain servers and other server management job(depende nsad ni sa imo company sometimes imo tanan ang trabaho hahaha)..
things mention above may vary to different company you are in..hehehe
to get more info just google it..