If you are connected to WiFi your phones data will come through your wireless network, not the cellular network. So any data you use will not be counted toward your data plan, which is unlimited so you really don't have to worry about using data, but a WiFi connection will probably be faster than the 3G connection. Data will not use your voice minutes anymore, plans used to use voice minutes as data a while back but Verizon changed that some time ago to have data billed separately.
If you are asking: when you are connected to WiFi and you make a call to someone does that use minutes? Then yes, you will still use plan minutes for calls even if connected to WiFi. As Kyllle said though, there are alternatives like Skype that will allow you to do VoIP and use your data connection to make calls without using your voice minutes.
So, if my goal is to reduce battery drain.... what services on the phone would require me to enable WIFI ?
Doesn't my cellular service provide the same connections to the internet/email/etc (but at a different speed, i assume). I'm sorry to be WiFi knowledge deficient here... but I guess i'm just not sure of the advantages of using it for the same functions that my celluar service would provide; or like I said- i dont know what functions I *have* to use wifi for (if any)
Again - my goal is to increase my (terrible) battery life ... [I'm reading tips for extending battery life on other threads too]
Basically just about anything the phone does with the exception of talking on it can use wifi if available. And having the plethora of functionality of the phone channeling through wifi wherever possible will extend battery life as using 3G for these functions consumes more power. The big one to save power is to go to settings/brightness and turn down the auto brightness slider to as low as what works for you. I'm at about 30% max auto brightness (I do leave auto brightness on, I just limit how bright it will adjust by changing this setting). Also note that the Facetime functionality uses wifi too. Once you switch to Facetime, your conversation is freed from your minute counter and is channeled through wifi.