sync - merge - what EXACTLY do these terms mean?

Howdy. I recently plunged into the iPhone world & I'm LOVING IT!!!!!!

So, what exactly does SYNC & MERGE mean? what will be the final results?

with my iPod shuffle, I'm able to "auto fill" from a folder of pre-selected podcasts in iTunes. After listening to the iPod for awhile, I reconnect the iPod to iTunes (on the computer) & the podcasts in iTunes are updated as with actions taken on the iPod. for example, iTunes on the computer will indicate wether or not I listened to these particular podcasts on the iPod or not, & will also remember where I left off. So it seems that the iPod updates iTunes on the computer.

But, ... this does NOT seem to be the same with the iPhone. It seems like when I plug in my iPhone to the computer & "sync", iTunes isn't updated as to what I already watched on the iPhone, like the iPod did. In order to update iTunes with what i already watched on the iPhone, I need to manually delete the vidcasts from the iTunes on the computer, which seems like a step back from my little iPod shuffle.

ALSO: if I change contact information on my iPhone, will this updated info be lost when I connect the iPhone to the Computer?

So what exactly does SYNC & MERG mean?

It seems that the iPhone reacts differently for different media. I'm very confused & I wish there was a place to understand all this without the generic "sync" word being used without explanation. does sync mean replace, update, ... ????

I've been searching around the net for this info for awhile, but all I find is these terms, without sufficient information as to what exactly MERGE & SYNC actually do.

Can someone suggest some place that explains this in the detail I'm looking for?

Thanks so much.

Jesse Luke