Please let me know can I use my existing vodafone sim in IPhone 3GS?

I like to buy Iphone 3GS phone, Currently I have Nokia phone having Vodafone Sim card.

So can I use same SIM and phone no with Iphone 3GS. Please let me know if I buy IPhone how to activate and use this phone. And what is factory unlocked means.

Basically there are two scenarios: Either you get the iPhone from Vodafone, then it's no problem of course and no need to unlock or you get the iPhone locked to a different carrier. In this case you will need to get it factory unlocked (having your IMEI marked as unlocked in Apple's database) in order to use it with your Vodafone SIM Card.

In case you require a factory unlock for your iPhone, you can either get this from any of the big online providers like http://www.cellfservices.com/ or in a local phone repair shop. However, be aware of unlocking via jailbreaks as this can permanently damage your phone and disables you from upgrading iOS.

Please visit any of your nearest Vodafone Stores to get a Micro SIM for an iPhone 3GS.

Regards,
Vodafone Customer Care.

Factory unlocked phone is a phopne in which you can use any carrier's sim card… As factory unlocked phones are costly people buy phones with contract like at&t, o2, orange, voda and other.,.so if you buy a carrier based iphone then you will be able to use sim of specific carrier which the phone belongs to. Like an at&t iphone can run only at&t sim, until it is unlocked via jailbreaking it. Jailbreaking the iphone provides u temporary unlock i.e your iphone will be locked after the update and you will have to again unlock it.
Also you can permanent unlock it via imei unlock service on ebay or in GAFAR and other iphone markets…

And yes you can use your current sim with iphone 3gs as iphone 3gs uses normal sim, iphone 4- micro sim, iphone 4s-micro sim, iphone 5- nano sim

All the iphones require a micro sim card which is available with all the telecom service providers. They may charge some fees for it; however that's the only option you have.