In case of iPad with 3g+wifi capability, you can use a 3G micro SIM Card also called the 3FF SIM Card which is quite small in size and you may need to remove it from your iPad to change the cellular network, below is simple tutorial to do it.
This 3G Micro SIM Card slot is a hot swap slot, you can remove it any time without terminating any application, nothing will happen as such only those apps which are using 3G will stop working when you remove the SIM from iPad.
Procedure To Remove the micro-SIM card From iPad
1. Insert the end of the SIM eject tool into the hole on the SIM tray. Press and push lightly the tool straight in until the SIM tray pops out. If you don’t have a
SIM eject tool, you can use the end of a paper clip.
2. Pull out the SIM tray and remove the micro-SIM card from the tray.