what does cellular mean?

You can compare iPad models here.

http://www.apple.com/ipad/compare/

Retina display gives the iPad a high resolution, super sharp screen image. It really is something to see. Colors are sharper and more vibrant. Text is super sharp and very clear even when you pich to zoom on the page. Videos and Photos look spectular on the retina display.

Cellular refers to the ability to connect to cellular towers for data transmission and you have ability to connect to the Internet even when you are not near a WiFi network. With a cellular iPad, you can be out in a car riding down the highway and still have Internet connectivity where as with WiFi only, you have to be within the range of a WiFi network that you can connect to in order to have Internet access.

A cellular data plan comes with a monthly cost in order to use the feature, but you can buy a cellular iPad and not activate the service until you need to or want to and you can just use it on WiFi for the time being.

You can read more about it here. This will give you a general idea of what 3G and 4G are about.

http://support.apple.com/kb/HT5205

Read the iPad comparisons to see the features that each model has.

Retina is recommended if you don't want to see pixels. Pixels by the way are small little quares used to create pictures, we see them always on computers. Anyway, retina just sets the screen resolution very high up so you can see circles as a circle and not see it like a blocky minecraft circle. It gives you good quality, and retina display is only for iPads 3 and up. You would want it if you like high resolution and no squares. It would be good if tplaying games. Also, retina display means pixels cannot be seen by a human eye, so, this will help for sharper text and pictures. It makes everything clear.

3G is when you don't need a Wi-Fi network to connect, and so is 3G. Those mostly come with phones. 3G and 4G is useful if you don't want to locate every single password outside, it will just give you Wi-Fi instantly.

Hope this helped!