What is a SiM card and do I need one in order to send email from my iPad?

when ever you have an wifi connection you can send emails

if you don't have a wifi connection then you need to find a carrier which support microSim cards and get a data plan with them which don't rob you too much

For further detail, a SIM card is a very small, thin, printed circuit card which you insert into a tiny drawer in your mobile device. It is your mobile/cellular account, with your personal details on it. So you can move it from one device to another. Before smartphones (like the iPhone), SIM cards also contained your Contact list. Since smartphones contain so much information now, the SIM card just identifies you to the cellular/mobile network. It means you can carry your phone number with you from one phone to another.

You can get a SIM card for iPad from any Apple Store or phone shop. Note that the newer iPhones and the iPads use what's called a "micro-SIM", which is smaller than a regular SIM card, so it takes up less room inside the phone, and they can stick more hardware in there for better performance. Make sure you ask for a "micro-SIM for iPad".

As Rudegar says, you don't need a SIM in your iPad if you always have WiFi access. But if you need to send email or messages outside a WiFi network, then you need a SIM, so you can access the cellular/mobile phone tower network.

You can get prepaid SIM cards which give you so much data for a certain period of time. You need to decide how much data you need, and how often you want to "recharge" it. The salespeople should be able to help you work that out.